Everyone is advancing theories as to the answer to this latest mystery. Perhaps they've landed on top of or inside of something else which is still moving. The scanner is covered with static, suggesting an unsuppressed motor or an electromagnetic field. Either way, it tells them little. Susan suggests going outside, but the Doctor hesitates. Barbara wonders why they ever leave the ship - it always leads to trouble. The Doctor is concerned that she's still upset over their adventures with the Aztecs, but she assures him she is fine.

This talk of past adventures gets Ian to thinking about how much they've been through together and how much they've all changed. Susan agrees. The Doctor sums it up: "It all started out as a mild curiosity in a junkyard. Now it's turned into quite a great spirit of adventure." Even their close calls haven't dampened their enthusiasm. Ian, Susan, and Barbara run down the list of places they've visited in the TARDIS. The Doctor adds a story about a quarrel he provoked with King Henry VIII. He and Susan were thrown in the Tower...where the TARDIS was hidden and they were able to escape. He realises, though, that this happened long before they met Ian and Barbara.

Turning his attention back to the problem at hand, the Doctor decides to go outside and investigate after all. Susan opens the doors. All the readings are fine; they have only the unknown to worry about. The answer to their mystery is solved almost immediately. They have landed on a moving spaceship. They are on the control deck and all of them can feel that the true mystery is just beginning. Something is very wrong here.

Susan closes and locks the doors and the group moves forward into the control area. There they find a crew of two, a man and a young woman. Both are slumped in their control chairs, unmoving. Both of them show no signs of life. There are no wounds evident and both look quite peaceful. Ian suggests suffocation, but the Doctor refuses to speculate. All of them are saddened by this tragedy. Susan wants to go back to the ship and Barbara worries that whatever killed the crew could kill them too.

The Doctor has noticed the crew members' wrist watches. They are the non-winding kind whose springs are recharged for 24 hours by wrist movement. Both are stopped showing 300. But the Doctor points out that even if both watches just now stopped moving, the crew members have been dead for 24 hours. But both are still warm. Something doesn't make sense. Despite this, the Doctor is inclined to leave. There's nothing they can do for the poor unfortunates and the ship seems a dangerous place to stay.

However, before Susan can unlock the TARDIS doors, the male crew member changes position in his chair and groans. They run to help him, knowing full well that he had no life signs whatever a moment ago. The man is barely conscious but very much alive. He uses all his strength to direct Ian to a shelf full of equipment along one wall. Ian finally locates the small metal box and hands it to him. He presses it against his chest and soon is fully alert. He asks Barbara to do the same for the girl. Thinking it futile, she does so anyway. To her astonishment, the girl comes to life and sits up with a groan. The man explains that the box is a heart resuscitator. They were not dead but in a deep sleep. The machine brought their heart rates back to normal.

The man introduces himself as Maitland, captain of this ship. The other crewmember is Carol Richmond. Both are from Earth. Ian and Barbara are pleased to see fellow humans, but it soon becomes clear that these two are from the far future; a time when Big Ben, even the city of London itself, no longer exists. It is the 28th century. Carol cuts through the pleasantries, urging the travellers to leave. Maitland concurs, saying there is great danger for them here. Ian and Barbara want to help and so does the Doctor, despite his assertion that he hasn't "an ounce of curiosity" in him.

Maitland explains that they are in orbit around a planet called the Sense-Sphere. The creatures on that planet - the Sensorites - have kept the ship from leaving. They have some sort of hold over the minds of the humans and have kept them prisoner. It is a strange form of passive hostility. While they have trapped the ship here, they have never actually harmed Maitland or Carol. In fact, they have even come to the ship periodically to leave food for the humans. Maitland urges them all to leave now, before the Sensorites decide to keep them prisoner as well. His fear is palpable. But unbeknownst to them all, it is already too late. A grey-gloved hand, not human, probes the TARDIS door unseen, settling at the lock. Soon the creatures produces a tool or a weapon - paddle-like with metal crosshairs in the centre - and aims it at the lock. Soon smoke is pouring from inside the lock.

Susan and Barbara can smell something burning but are unable to place its location. Ian wonders if there isn't something they can do for the two astronauts and Susan suggests taking them along in the TARDIS. Maitland flatly refuses to leave and Carol explains that they must think of "John" as well as themselves. Before anyone can ask her what she means, Barbara mentions the burning smell again. It has grown stronger. Nothing has shorted out, so what can it be.

At the TARDIS, the creature's work is done. It slides out the entire lock barrel from the door and disappears into the shadows.

Barbara pinpoints the direction of the smell and she and Ian go to investigate. The Doctor inquires about this John and finds that he is an additional crewmember. The other seem reluctant to divulge much more. The Doctor decides to accept Maitland's pleas to leave the ship and he bids the astronauts goodbye. He and Susan head for the ship, joining Ian and Barbara who are still seeking the source of the burning smell. However, it is Susan who finds the answer first, seeing the missing lock. The entire opening mechanism has been removed from the door and it is most effectively locked. Ian and Barbara cannot believe it, but the Doctor assures them there is no way to get inside. They too are now prisoners of the Sensorites.

Suddenly the ship begins to shudder and rock. Carol screams it is the Sensorites taking control. The engines have been activated. The travellers race to the control area where Maitland sits paralysed with fear. The Doctor takes charge, checking thrusters and stabilisers. Maitland's defeatist talk is of no help and the Doctor silences him. He engages the ship's stabilisers and the rocking stops. However, the damage is done and the ship is now heading directly for the Sense-Sphere. Maitland can do nothing more than point the Doctor to the controls he wants. Carol has recovered herself somewhat and reports that they are gaining speed and Ian finds a readout showing 19 miles to point of impact. They are on a collision course.

The Doctor works the controls rapidly, preparing to reverse thrust. Carol recovers herself more and - with Ian's help - activates the thrusters twice on the Doctor's command. It does not work; velocity is still increasing. The Doctor then decides to change tack and actually increase their speed, forcing Maitland to do the work for him. They are virtually throwing themselves at the planet. Miraculously, the ship veers away from the planet at the last minute and they are safe in a new orbit. Maitland, overcome with fear, laments his inability to act in the face of the danger.

A little while later, Maitland is feeling better and the Doctor is pleased with their escape. This was not truly an attempt by the Sensorites to kill them, just to frighten them and extort control. They had no intention of letting the ship crash, hence the last-minute reprieve. Somehow, the Sensorites can control Maitland and Carol's minds through fear. But the four travellers are immune to it so far and Carol was able to resist. Maitland's control was taken from him, perhaps through his lack of hope. The Doctor, though, is still puzzled as to why the Sensorites don't seem determined to kill them.

Ian inquires about food and Susan and Barbara show him the unappealing space rations they are preparing. But they need water and ask Carol where it is. She points vaguely toward the back corner of the deck. They nod and plan to leave in a moment. The Doctor asks the astronauts if they have seen or spoken to the Sensorites. He learns that only John, their mineralogist, has. But Carol and Maitland are adamant that the Doctor must not speak to him, and adamant in their refusal to say why.

Susan and Barbara go off looking for the water, but pass by the small container on the shelf and instead approach a bulkhead door. They believe the water to be on the other side. After a short search, Susan finds the light beam opening mechanism and breaks the beam. The door slides upward and they go through. However, there's nothing but blank metal corridor before them. No sign of water. They head for an interior door ahead on the right. Neither of them notice the figure standing in the shadows behind them. It is a man, wearing a uniform like the other astronauts, but he is unkempt, has worn and haggard features, and his hair is snowy white. His eyes are blank and very dark, staring forward as if seeing nothing. He passes his hand across the light beam mechanism several times and the bulkhead door closes, sealing out most of the light from the passage. This is the mysterious John. He moves off after Barbara and Susan, keeping to the shadows, shambling zombie-like as he goes.

Susan and Barbara find a cabin beyond the door. It is packed with books, papers, and personal belongings. Susan is quickly rummaging through them. Barbara can see there's no water here and wishes to leave immediately. However, she hears a noise outside that stops her cold. Something is in the corridor.

Ian and the Doctor have had no luck in convincing Carol and Maitland to let them talk to John and the Doctor finds their silence very suspicious indeed. So intent were they in this that no one noticed Susan and Barbara leaving. Ian wonders where they are. Carol and Maitland realise in a second that there is only one way they could have gone and they rush to the bulkhead door. It has been locked from the other side. Shouting that the girls are in great danger, they lead Ian to a second bulkhead. It opens, but the interior door which leads to the area where the girls are is jammed. There is no way in to them. Carol and Maitland are prepared to give up, but Ian shoves them aside to pound on the door. He demands to know the nature of the danger, but again the crew fall silent, further fuelling Ian's anger.

John shambles through the doorway staring blankly forward. Barbara and Susan stay close together, frightened by this spectre but keeping their response in check. John advances toward them and holds up his hands, still moving very slowly. When he reaches them, he falls forward. They dodge and he collapses on the bunk. It doesn't seem an aggressive move, but the women are startled nonetheless. John no longer blocks their escape and so they run for the corridor and the bulkhead door. It will not open and they realise they are trapped.

Barbara leads them to a second room, a storage area, and tries to move a locker in front of the door. But it is too heavy. John, back on his feet, shambles into the room looking more pathetic than ever. He doesn't even seem to be aware of the girls. His face contorts in a grimace of pain and he puts his fists to his temples as if trying to keep something out. After a moment it seems to subside and John leaves the room, supporting himself against the walls. He still takes no notice of Susan and Barbara.

Maitland and Carol debate what to do. It becomes clear that they have not seen John in some time, having locked him back in the rear of the ship on his own. Carol has clearly not been pleased with this arrangement and pleads with Maitland to try and get in to him. She explains that her hope for escape has returned, especially since she was able to overcome her fear earlier and regain some control of herself. Maybe they can help John now when they weren't able to before. Maitland says it's too dangerous to go inside, but Carol realises he is trying to keep her away from John. The last time he saw her, John didn't even remember her name. Maitland's sure that John has deteriorated even further on his own and that Carol will be crushed when she sees him. But her sadness convinces him to at least try. He goes to fetch cutting equipment to bypass the bulkhead lock.

Ian gently asks Carol for more information and she explains that John is her fiancé. The Sensorites attacked his mind far worse than herself and Maitland. She watched him as he got worse and worse, his mind destroyed and consumed by fear. He will be frightened of the girls as they are strangers and Carol fears he may become violent.

However, at this moment John is only slumped weakly against the corridor wall. Susan and Barbara fail to notice him until he starts to move toward them. Hearing the steps, they back hurriedly away, but they reach a dead end with a locked interior door. They are trapped. John continues to shamble toward them, looking like a madman. He suddenly clutches his head and Susan screams. But instead of attacking them, he falls to his knees and buries his face in his hands. He begins weeping.

Barbara and Susan and taken aback when the man looks up at them imploringly, like a lost little boy. He asks who they are. He thinks Barbara looks like his sister and he hopes they've come to help him. Barbara makes a move to go to him but Susan holds her back. This only seems to make him sadder. He mutters something about "4 years" and admits that he is ill. This is too much for Barbara and she kneels down with him. He curls into her arms, as if looking for protection, and continues to sob quietly. Both women wonder what could have happened to him.

Maitland goes to work on the door with electromagnetic cutting equipment but it is a slow process. Carol, Ian, and the Doctor stand nearby. Carol says it's been months since they've seen John. He became so unstable and dangerous due to the Sensorites' attacks that they had to confine him. Hearing this makes the Doctor even more impatient to get through the door. He becomes cross when Maitland stops and switches off the equipment. He listens to another sound still audible and growing louder, high-pitched whine. Carol tells them it is the sound of the Sensorites' spacecraft. They are very near.

Carol returns to her control panel and the Doctor takes Maitland's seat. They all watch the main window, looking for moving lights which herald the arrival of the Sensorites. Ian wishes to prepare some sort of defense, but Maitland says it's hopeless. Soon two oddly-shaped moving lights appear some distance away, moving toward the ship. Those craft are how they got here to steal the TARDIS lock and, presumably, take it away to the Sense-Sphere. Now they're coming back. But why? To take over the minds of the newcomers, or to kill them?

In the corridor, John and the others become aware of the Sensorite presence as well. Susan and Barbara can hear the noise, but John appears to "feel" their arrival in his mind. He sits up, prepared to defend them even though he is clearly very weak.

Ian suggests trying to move the ship, but Maitland thinks that useless. He tells Ian they won't kill them; they've never tried it yet. Ian feels that the collision course was the harbinger of harsher treatment, but an angry Carol knows it is just more of the same war of nerves.

The Doctor hushes them all. The engine whine has stopped, and he hears a new sound - a sort of scratching. The instruments all go blank, just like with every arrival. Ian insists on finding some way to defend themselves, but the Doctor rebukes him. Those are creatures of the mind, not brute force. Their intelligence must be their defense... and their offense. Suddenly all noise ceases and an eerie silence falls over the ship. Maitland's fear is evident on his face.

The Doctor warns Ian to keep calm in order to resist the mental power of the Sensorites. It must be working as he and Ian are fine. Maitland and Carol, however, have frozen into immobility. Their fear has given the Sensorites control over them again. The Doctor manages to bring Maitland back to himself by redirecting his thoughts, focusing him back on cutting open the door. Soon he goes back to work, ignoring the Sensorites outside.

In the inner corridor, Barbara and Susan try to get John to unlock the door and let them out. John refuses, saying that all their friends outside are dead and that only he can protect them. Suddenly he puts his hands to his head again, as if fighting something. He falls to his knees and begins responding to unheard questions. Susan realises the Sensorites are talking to him telepathically. John refuses an order to frighten the girls and fights harder. It is clear that he is in pain, sobbing about the cacophonous voices in his head. Susan realises he is being ordered to harm them and wonders if he can resist.

Ian becomes ever more frustrated over the slow pace of Maitland's cutting. He's worried sick for Susan and Barbara, assuming John to be some sort of mad monster. The Doctor tries to calm him, saying that an emotional and confused mind is easy to control. Carol, also calm and back in control, reports that the Sensorites have entered the ship through the loading bay. From there, they can reach the section where Susan and Barbara are - to say nothing of their puppet John. Ian rushes to the bulkhead door and pounds on it in frustration.

Ian's pounding does carry into the rear section. Susan and Barbara's spirits brighten, but John tells them the Sensorites are still giving him orders. He is resisting, but is becoming emotionally unstable, which they fear will make him lose control. They try their best to calm him and that they are his friends. This seems to make him stronger and he reports this to the Sensorites.

On the other side of the door, the cutting continues.

In another part of the rear section, two Sensorites arrive. They are smallish creatures, mainly humanoid with two arms and two legs, but their feet are simply large round flippers with no digits. Their bodies are soft and rounded, their heads orange and bulbous. Both have wispy white facial hair which obscures the bottom part of their faces and two enormous dark eyes. Both are armed with the paddle-like ray objects and are wearing soft, grey, featureless unitards.

John is quiet now, as if receiving no signals. But Susan is sure the Sensorites will "attack" him again. She suggests trying to use their own minds to defend themselves. She tells Barbara the story of the intelligent plants she and her grandfather encountered on the planet Esto. They communicated through thought transference and could detect other minds which approached. John suddenly stands, frightened, announcing that the Sensorites are here. Susan and Barbara decide to combine their thoughts, centering on the phrase "we defy you" in order to try and break in on the Sensorites' communication. They do so on the count of 5, concentrating hard.

In a storage room, the Sensorites suddenly collapse in pain, holding their heads.

After a long moment of concentration, Susan herself collapses under the strain.

Back in the control area, Maitland has succeeded in raising the door a fraction. Brute strength - with help from a determined Ian - raises it up the rest of the way. They rush inside.

Later, everyone is safe and calm. Susan has explained everything and the Doctor is hopeful that they now have a way to defend themselves. Although, Susan explains, she was not "attacked", but only collapses when she heard hundreds of voices all talking at once. The Doctor takes her to task for doing something so dangerous; it was only her fearlessness which kept her from being harmed.

John is sleeping peacefully now. Maitland was taken aback at John's white hair and haggard appearance. He still wonders why the Sensorites singled John out for this extreme treatment. Ian has overheard John mutter the phrase "the dreams of avarice" and acquaints the Doctor with the Earth phrase about riches. Ian thinks that John - the ship's mineralogist - must have discovered something that the Sensorites wished to keep hidden. They silenced him and kept the others prisoner. Now the travellers are caught up in the whole thing. The Doctor suggests a simple way to find the answer: talk to the Sensorites.

In the rear part of the ship, one of the Sensorites uses a small disc pressed to his forehead to communicate with the Sense-Sphere. He speaks to their leader, the First Elder, who is interested in making contact with Susan, the voice they all heard in their minds. But the two Sensorites are instructed to proceed with caution as these newcomers are more intelligent than the astronauts and are not afraid. The plan for now is to observe them only, but any offensive moves will be met with deadly force.

The Doctor examines John's work station and learns from Carol that the first Sensorite attack came just after John finished his preliminary scans. The Doctor hands the spectrographic analysis to Ian, who only sees the usual collection of minerals. Ian slips into teacher mode, starting to instruct Susan about spectrographs, but she knows all about it and moves off. Ian finds nothing of particular value indicated on the analysis.

Carol tries to find out more about the travellers, sensing that they don't really have any true destination. Barbara explains that the Doctor is trying to get her and Ian home, but in a roundabout way. For her part, Susan thinks it's better to "travel hopefully" than to arrive. For Carol, any destination would be preferable to being trapped in an endless orbit.

The Doctor has found no clues, nor has Ian. Maitland has been studying the data since the first attacks and has found nothing either. The Doctor takes the spectrographic analysis for himself and sits down to examine it more closely. Barbara puts the pieces together. John discovered something of value on the Sense-Sphere - although no one knows what - and he became so excited that his mind opened up and broadcast it to the telepathic Sensorites. For that, they attacked him.

After a moment's study, the Doctor finds the answer: it is the mineral molybdenum. The planet is rich in it, but its lines are spread out over the graph and difficult to detect. It is an alloy of steel with a very high melting point and is critical to spaceship construction. It is no wonder to him why John was so excited. The Sense-Sphere is a veritable gold mine.

Suddenly Maitland and Carol collapse in pain, gripping their heads. The Sensorites have heard them; they are close by and attacking their minds. Ian and Barbara decide enough is enough and head into the rear section to find and confront the creatures.

Once through the door, they move slowly and quietly, trying to sneak up on the creatures. In the first cabin, they see nothing. The second contains John, sleeping peacefully. The third cabin is also empty, but it contains the doorway to the other section. Cautiously, Ian opens it and they both go through into the other corridor. Still no sign of Sensorites. All is quiet and still. They find a shorter corridor which ends in a door. Ian makes sure Barbara is ready to go before opening it.

They go through and find themselves in a large, seemingly-disused room. They move a little further in and suddenly catch sight of their attackers - two Sensorites. Both pairs stand still for a shocked moment, but soon the creatures begin to advance slowly. No words are spoken. Ian and Barbara retreat, Ian looking around for some sort of weapon. He finds some sort of metal tool like a spanner and grabs it. The Sensorites take a defensive step backward, but when Ian doesn't press the attack, they advance again. Barbara opens the door into the corridor and they slowly retreat through it. However, once through, it dawns on them that they cannot lock it. Ian sends Barbara back to the control deck to find out how. Ian will hold the Sensorites off.

The Sensorites come through the door, standing nearly face-to-face with Ian. Still they say nothing and make no overtly offensive moves. Ian slowly retreats and they slowly advance. He arrives at the door to the pass-through and tries to open it, but he doesn't break the beam and it stays shut. He must raise the tool above his head again to force the creatures back long enough for him to try again. This time the door opens and Ian backs through it. He's running out of places to retreat and possible doors to lock.

Barbara badgers a woozy Maitland for information on locking the doors, but he's unable to think clearly and is of no help. The Doctor talks Barbara into asking John and she goes to find him. John is surprisingly responsive and moves quicker than Barbara has ever seen.

They meet up with Ian who seems just about at the end of his tether. He raises his weapon and looks like he means business. But Barbara stops him. She doesn't think force is necessary; the Sensorites haven't actually attacked. She hurries John to the locking mechanism and the door slams shut. John thinks it will keep them out.

Ian and Barbara are confused as to why the Sensorites didn't attack. The little creatures don't even seem capable of aggression really; they actually seemed as frightened as they were. They start back to rejoin the others when Ian remembers the bulkhead door on the other side of the control area. He runs through to it and locks it. He returns to the first bulkhead door just as John and Barbara get through. The three of them pull the door down and bolt it. Now they'll see just what the Sensorites can do.

In the rear section, one of the Sensorites produces his paddle-like weapon and directs it at the light beam control of the door. In a second the door is unlocked and they are through to the other side. They make their way to the bulkhead door, but when one of the creatures makes to unlock this as well, his comrade stops him with a shake of his head.

On the control deck, the Doctor is trying to work out their next move, which he thinks should be making contact with the Sensorites. Maitland and Carol are recovering from the attack. Suddenly, Susan speaks out loud, answering questions seemingly from nowhere. She reports to her friends that the Sensorites have contacted her telepathically and asked for a meeting. While he is taken aback by this, the Doctor agrees, on the condition that the Sensorites will not harm anyone. He promises to fight back if they attack.

Susan goes silent and appears to be communicating. Without a word, she moves off toward the bulkhead door and unbolts and opens it. The two Sensorites are waiting for her just on the other side. They step through into the control area, intending to speak to the Doctor.

However, the Doctor is first to speak, demanding to know why this ship and its crew are being held here. The Sensorites, speaking softly and with raspy voices, say that none of them can ever leave the Sense-Sphere due to their discovery of molybdenum. Ian assures them they are not interested in the mineral wealth of the planet, but the Sensorites refuse to trust them. Once before, Earth people came to the Sense-Sphere and caused them a terrible affliction. All of the humans must come down to the planet and stay there; a special quarantine area has been prepared. These two have come to take them down.

The Doctor flatly refuses to stay, on the Sense-Sphere or in the ship. While John, Carol, and Maitland may be paralysed by the minds of the Sensorites, he and his friends are not. They promise to fight if forced. However, the Sensorites stick to their demand. They still propose to take all seven to the Sense-Sphere, but they do not wish to harm anyone. Ian says they too do not wish to harm the Sensorites, but will do so if forced. Stalemate.

The Doctor orders the creatures to return the lock of the TARDIS. His anger bursts forth and his voice rises and soon he is shouting at them. They cower back, as if the noise is too much for them. Once they recover, they retreat through the bulkhead to confer and decide what to do.

This seems ominous to Ian, who wonders what else they may have up their sleeves to break the stalemate. The Doctor fears it may have something to do with Susan, with whom they clearly have a link. They might try and control her, just like Carol and Maitland. Despite the astronauts' susceptibility, however, the Doctor now knows the Sensorites are not invincible. He points out their large dark eyes, the exact opposite of a cat's eyes. Their pupils were almost completely dilated in the bright light. In the darkness, they will likely be nearly blind, and therefore frightened. He plans to use this against them. The Doctor thanks Ian for his admiration on this deduction, even though Ian hasn't said a word. The Doctor reveals that he, too, has telepathic capability and sometimes knows what Ian is thinking. He laughs to himself at Ian's astonishment.

Susan receives a message from the Sensorites. Her answer: "I don't want to go." But the message is insistent and after listening a moment longer, Susan agrees to the unheard request and tells all of her friends not to move.

Susan walks to the bulkhead door, where the Sensorites join her. She tells her friends that this is the only way to solve their problem. She has agreed to go down to the Sense-Sphere with them. Otherwise, they will all die. Before her friends can stop her, Susan steps through the bulkhead and the Sensorites close it fast. She is now their prisoner...

The Doctor and his friends fear for Susan's life. Maitland and Carol fear for their own should these strangers intervene. Ian's insistence wins out and he and Barbara open the bulkhead. They plan to test the Doctor's theory about the Sensorites' eyes as they go after the retreating creatures. Susan tries to reassure her friends, but they insist she stay behind. The determination of these humans concerns the Sensorites and they recall the earlier humanoids who visited the Sense-Sphere. Again the creatures threaten to fight if provoked.

Susan tries to intervene, saying that she wants to go with them; to promote trust between them. But the Doctor joins the discussion, turning up the heat. If she goes with them, the Sensorites will have all the advantage and refuses to let her go. The Sensorites must prove trust as well as demand it. The Doctor's words anger Susan and they argue. She says she is tired of being treated like a child, but the Doctor commands her to leave the Sensorites and come to him. After a tense moment, she does so.

The Sensorites brandish their hand rays, planning to stun the humans, but Ian pulls a switch and the lights in the corridor go out. The Sensorites cower in fear, blind in the darkness. They drop their weapons, whimpering. The Doctor was right. Ian grabs their weapons and the Doctor sends Susan back to the control deck. The Sensorites beg for the light to be restored and the Doctor complies. He is now in charge, although he promises only to use the creatures' weakness in his own defense. He demands the TARDIS lock and becomes furious when the Sensorites say they must call down to the planet for new orders. He stalks off back to the control deck to wait, his anger at boiling point.

Barbara watches him go, concerned. She's never seen him so angry before, more at Susan even than the Sensorites. Ian wonders if the Sensorites hypnotised Susan to cause her to defy her grandfather, but Barbara knows better. His granddaughter is simply growing up.

The Doctor manages to tone down his anger somewhat as he speaks to a sullen Susan. He knows she was only doing what she thought was right, but he believes that his age and experience makes him better qualified to make these choices. Susan is angry at being treated like a child, especially in this case where she has a direct connection to these creatures. The Doctor agrees that her connection is valuable, but this does not give her the right to make a unilateral decision for the group. Susan refuses to have her input brushed aside.

The Sensorites arrive on the control deck, concerned over the misery they can read in Susan's mind. The Doctor dismisses this angrily, blaming them for starting the argument in the first place. Susan relents before the Doctor really lashes out at the creatures. He hugs her, saying that her input will not be ignored. They must all work together to get the lock of the TARDIS back.

The Sensorites report that they have spoken to their leader, the First Elder, and he has agreed to hear the Doctor out. Pleased at this, the Doctor pushes for a face-to-face meeting to arrange the release of the spaceship as well. Freedom, he says, is the only treasure they desire, not mineral riches.

Meanwhile, Carol has been tending to John. He suddenly sits bolt upright, complaining pitifully about the voices in his head. They are constantly imploring him to forget what he knows. All he wants is silence in his head. Carol tries to comfort him, but her heart is breaking anew. He still does not know who she is - all he can tell is that she is good. Maitland arrives to find Carol in despair, certain that her fiancé would be better off dead. Maitland tells her that she and John are to go down to the Sense-Sphere with the Doctor and some of his friends. The Sensorites believe they can cure John, but Carol's despair is absolute.

Barbara seems cheered at the prospect that John can be cured, but Ian is concerned at the time it will take. Barbara and Maitland must stay on the ship as a safeguard while the others go down to the planet. The Sensorites will cure John to prove their trustworthiness and Ian must accept splitting up. Barbara forces him to agree, sure she will be fine. A spacecraft will soon arrive to carry them through the vacuum of space.

The Doctor joins them and questions one of the Sensorites. He learns that the discs they wear allow them to communicate telepathically, especially over distances. Otherwise, they speak vocally to one another. The Doctor badgers the Sensorite to explain why they attacked Maitland and the others and took over their minds. He learns that the previous group of humans, 5 men who arrived nearly 10 years ago, were welcomed to the Sense-Sphere. But it was clear that all they wanted was the mineral wealth of the planet. The men quarrelled among themselves and split into two groups. One group stole their spaceship and blasted off, but the ship exploded. It is believed that the other group hid aboard and caused the explosion while fighting for control. Ever since that day, a strange disease has been killing the Sensorites in increasing numbers. The Sensorites believe that humans are dangerous in every way.

But the First Elder senses great knowledge in the Doctor and wishes to take a chance in meeting with him. The Doctor thinks he may have found the bargaining chip he needs to get the TARDIS lock back. Shortly the Sensorite transport arrives and the Doctor and Ian bid goodbye to Barbara. Barbara takes Susan aside to reassure her that she is on the girl's side. She reminds Susan that her grandfather loves her and is only looking out for her. The Doctor comes and fetches Susan and they are ready to go. One Sensorite will also stay on the ship with Barbara and Maitland. The other conducts Ian, Susan, John, Carol, and the Doctor to their transport.

Down on the Sense-Sphere, an emergency meeting is in session. The First Elder's decision to allow the humans to come to the planet is being disputed by the Second Elder. He fears an acceleration of the disease the first humans brought, or perhaps a different and even more deadly disease. But the First Elder's word is law and he will not be swayed. He explains that he plans to press the Doctor to cure the disease before any favours are granted. But it is more than that. The First Elder is an open-minded man of peace and wisdom. While the Second Elder sees humans as loud and ugly creatures, not unlike animals, the First Elder sees beyond appearance. The locking mechanism they retrieved from the Doctor's ship is an advanced electronic marvel and the Doctor says his ship can travel through time. These creatures are very advanced, but they have made no offensive moves. He wishes to build trust and hopes that much can be gained for both sides.

Despite misgivings, the Second Elder agrees, moved by the First Elder's well-spoken arguments. The City Administrator, also in on the conference, agrees as well, but in such a way as to worry the First Elder. The City Administrator merely defers to his leader, expressing no opinion of his own. The leader chides him for being simply a "yes-man" and departs.

It soon becomes clear that the City Administrator does not agree at all with the First Elder's decision, despite what he said. He thinks he has an ally in the Second Elder and reveals to him that he does not trust the humans. He has had a weapon called a disintegrator brought to the palace on his own authority. He says he is merely being cautious in protecting the First Elder, but the Second Elder labels him presumptuous. He forbids the City Administrator from doing anything further until he himself has considered the matter further. But once the Second Elder is gone, the City Administrator reveals the extent of his plans. He will not wait and the Sensorites will not be safe until all the humans are dead.

The Doctor and his friends arrive and are conducted through the capital city toward the palace. They have seen protests against their presence as they've walked, people who fear another outbreak of disease. The Doctor hopes to talk to them, to assure them the disease was not a deliberate act of violence and that a cure may be found. However, their Sensorite guide forbids any of them to speak to members of the lower caste. He explains there are 3 classes - the Elders, the warriors, and the Sensorites. It helps to define each individual's role in life, and all are happy. The Doctor, Ian, and Susan are understandably sceptical.

The Doctor and Ian follow their guide into the palace immediately, but Susan hangs back with Carol and John. John has become agitated, saying he can sense an evil mind very nearby. Susan knows that his mind is open due to the Sensorite attacks and that he can sense these things. Unfortunately, he has trouble articulating any more. The three of them enter the palace.

In a secret anteroom, the City Administrator oversees the City Engineer as he finalises set up of the disintegrator. Its control console takes up one corner of the little room. Soon all is in readiness and the firing key inserted. The City Administrator provides precise coordinates for the seats assigned to three of the humans in the First Elder's reception room. The disintegrator is soon beamed and ready, aimed at the creatures' hearts. Only 1 switch remains to activate the beam. Electro thermocouples track their movements. As soon as the humans are seated, the beam can be activated.

Formal greetings are over and as a show of good faith, the First Elder decrees that John will be cured. The process will take time, but it can be done. John is conducted out, with Carol's help, but he pauses to gaze at the First Elder. John declares him a good man, but still senses an evil mind somewhere near. Still unable to pinpoint it, he falls silent and allows himself to be lead away.

Ian thanks the First Elder, but the Doctor still harbours some anger - it is the fault of the Sensorites that John was harmed to begin with. He raises his voice, causing the First Elder to shrink back in pain. The First Elder cautions that the Doctor should not jump to conclusions and asks them all to sit.

In the anteroom, the City Administrator is prepared to give the firing order. But just before the Doctor, Ian, and Susan sit down, the Second Elder bursts into the room. He has found out about the City Administrator's plans and orders them stopped. These humans are reasonable and friendly - they pose no threat. He forces the Engineer to turn off the disintegrator and he takes the firing key into his possession. He leaves with a stern warning to the City Administrator do not question authority or disobey orders again.

Unaware of their close call, the Doctor and his friends sit and listen to the First Elder's explanation as food and water are set out before them. The First Elder says that when John discovered the molybdenum, he pictured in his mind a fleet of spaceships coming to plunder the Sense-Sphere and devastate their planet. The only humane way to stop this was to put the humans to sleep telepathically and to keep them prisoner. However, in John's excitement, he had no reserve, no defense against the full power of the Sensorites' voices. His mental damage was an accident.

The Doctor considers this, starting to drink the water before him. However, the First Elder stops him. This is not the "crystal water" the Elders drink. It is common aqueduct water provided by the City Administrator as an affront to the guests. A servant is sent to fetch crystal water - a pure mountain spring water said to possess special qualities - for the humans. Ian is thirsty and takes a small drink of the common water while awaiting the other. Susan eats some of the delicious fruit. The Doctor, however, is ready to talk business.

In the anteroom, the City Administrator seethes. He is certain the First Elder is sacrificing their entire planet to these smooth-talking humans. The Engineer agrees, pledging loyalty to the City Administrator, despite his advocacy of overthrowing the First and Second Elders if need be. The time for action is not far away and the City Administrator welcomes this ally.

After some small talk about the governing structure of the Sensorites, Ian asks to hear more about the disease afflicting the people. An involuntary cough overcomes him as he asks the questions, but he ignores it and presses on. The First Elder says the disease is untraceable and is always fatal, resisting all efforts to cure it. It strikes all castes except the Elders, but afflicts individuals of all ages and occupations. The Doctor ignores another coughing fit from Ian to make a proposal: if he can cure the disease, he wishes the lock of the TARDIS back.

Before the First Elder can answer, Ian coughs again, this time much more severely. He grabs his throat, saying it is very dry. He can barely speak. The First Elder looks on worriedly and the Doctor knows what is happening: this is the first stage of the disease.

The Doctor goes to work immediately, gathering information about the disease. He learns from the First Elder that it is not contagious and has never affected the Elders. He notes the rapid buildup of Ian's body temperature and wonders if it is an airborne germ, even though it doesn't really seem like a disease at all. The only thing Ian did differently than the others was to drink the aqueduct water. Even though not everyone who drinks the water succumbs to the disease, the Doctor is reasonably certain this is their culprit. However, he thinks the symptoms are more like poison than a disease.

The Doctor sends a servant to fetch sodium chloride and the First Elders gives orders that Ian is to be made comfortable here in the reception room. He is much distressed by this tragedy. The Doctor orders that no one is to drink the aqueduct water until further notice and asks to work with the Sensorites' scientists. The First Elder agrees to all this, warning the Doctor that he has no more than 3 days to cure Ian before the disease kills him. The Doctor thinks this ample time and assures him that with the chemicals and equipment in the TARDIS, he will be able to end this disease forever. At this, the First Elder hesitates, preferring to speak to the Second Elder before returning the TARDIS lock. The Doctor is angry at the delay, but must wait as the First Elder goes off.

The servant returns and the Doctor dissolves some of the salt in water and feeds it to a barely-conscious Ian. He is trying to keep himself busy and to relieve Ian's suffering as much as possible, but his mind is occupied by a more serious question: will they let him have access to his ship?

The Second Elder is disinclined to return the TARDIS lock, despite the First Elder's trust in the Doctor. The Second Elder suggests Ian may be feigning illness for this very reason and that the Doctor may leave in his ship to fetch an army to take over the Sense-Sphere. Despite the First Elder's instincts, the Second Elder has given him doubts. He must continue to weigh the matter.

In the laboratory of the Chief Scientist, work is underway to cure John. The human sits slumped in a chair with a series of wired leads on his head. The City Administrator bursts in, angry at the time being wasted on this worthless human. The Scientist is only following orders, but the City Administrator thinks John would be better off dead. However, the Second Elder arrives and overhears this, chastising him again for questioning authority. A rant from the City Administrator about ensuring security only makes him sound more unbalanced.

The City Administrator learns of Ian's illness and of the Doctor's belief that the water supply is tainted. He is certain this is a lie, meant to undermine the Sensorites' faith in their basic infrastructure. The Second Elder had not considered this; and when John utters the word "evil", he is even more convinced of the humans' treachery. He hurries off to consult with the First Elder.

But John was speaking of the City Administrator. His open mind has finally focused in on the evil presence he sensed earlier. He tries to call out to the Second Elder, but he is already gone. The City Administrator taunts John and his weak mind shuts down. He passes out just as Carol arrives. Carol mistakes the City Administrator at first, prompting an angry response. But to her, the Sensorites all look very much alike. It is only by their badges and sashes of office that she can really tell them apart. This notion is entirely new to the City Administrator, and it seems to give him an idea. He hurries off as Carol returns her attention to John.

The Doctor is angry at the First Elder's continual delay in his decision. If Ian dies, it will be his fault. The Second Elder arrives and a hurried telepathic conference follows. Susan tries to overhear, but her mind is shut out. When it is over, the First Elder decides to refuse the Doctor the means to enter his ship. He must use the facilities her in the city to prove his poisoned water theory. The Doctor's fury grows and he shouts at the Sensorites, forcing them to cower back in pain and further antagonising them. But despite his ranting, the Doctor knows there is nothing he can do.

The Doctor goes to check on Ian. He is awake but only just. His throat is sore and he is very giddy, but has no other pains. The Doctor dodges his questions as to what's wrong with him and leaves Susan to act as nurse. The Doctor is ready to be conducted to the laboratory. He only hopes it isn't too late.

The City Administrator and the Engineer conspire in the courtyard of the palace, learning that the Second Elder gave the firing key of the disintegrator to the Chief of Warriors. It will be difficult to retrieve. The First Elder passes by and their talk ceases. He reveals that he has given the Doctor access to the laboratory before moving on. This fuels the City Administrator's anger anew. He believes that the First Elder's weakness will deliver them into the hands of the deceitful humans. He orders the Engineer to find the Second Elder and bring him to the disintegrator room. He reveals a plan to assume the Second Elder's sash of office and to impersonate him. He plans to match the humans' cunning with cunning of his own.

The Doctor has quickly taken charge of the laboratory and is outlining his theory to the Chief Scientist and his assistants. He believes there is not a disease but atropine poison in the aqueduct water and he outlines the symptoms in case the Sensorites are unfamiliar. They mirror Ian's symptoms. They must isolate the exact type of poison and then find the cure. But the Sensorites tell the Doctor that the water has already been tested and nothing was found. When the Doctor wonders why only some of the people become ill - when the aqueduct water has only one source - he learns that there are 10 different water districts in the city. Clearly the poison enters the water in the city, not at the source. Samples must be taken and tested from all the districts. And time is of the essence.

While Ian fights for his life, becoming more and more delirious, the Doctor and the Sensorites sample all of the city's water. District after district tests negative for contaminants...until they reach the water from District Eight. There the Doctor finds what he suspected all alone - atropine poison.

The Second Elder relays the good news to the First Elder. The cure has already been identified and the Doctor will shortly have it synthesised. The Second Elder goes off, bearing congratulations from the First Elder, but first he must keep an appointment. Susan passes on the news to a groggy Ian.

Unfortunately, the Second Elder's appointment is in the disintegrator room. There, the Engineer and another loyal Sensorite hold him captive while he is questioned by the City Administrator. He has the Second Elder's family group under guard as well and urges the Second Elder to cooperate for their sake. The City Administrator learns of the poison and the cure, but believes this is all a ruse by the humans. Ian is only feigning illness and the so-called "cure" is a poison which the devious Doctor will administer to kill the Sensorites. He removes the Second Elder's sash of office and puts it on himself. By impersonating the Second Elder, he plans to suppress this drug before it is administered... to Ian or anyone else.

While the Doctor awaits the antidote, he checks in on John. Carol says he is improving but still reverts sometimes to his confused state. The Doctor is sure it is only temporary. The Chief Scientist brings him a phial of the antidote, which the Doctor despatches to the First Elder. He instructs the Scientist to begin producing it in bulk.

The Doctor is sure they will soon be off this planet now and on their way. But first John's cure must be completed and that will take some time. The Doctor turns his mind to the source of the atropine, wondering how the poison source can move around the city, appearing in different districts at different times, seemingly at random.

John suddenly begins murmuring about enemies and plots against them. Carol thinks it is delirium, but the Doctor believes he is more lucid than that, especially when he seems to remember his own name for the first time. The Doctor instructs Carol to take careful note of everything he says. He himself is going on an "expedition". He is vague as to its nature and location, but believes that it will finally end the danger for them here.

Out in the courtyard, the Engineer fears that the City Administrator's plan will not work. But the City Administrator assures him the First and Second Elders are rarely seen up close by many people. The sash will be enough for their purposes. In fact, it works almost immediately as the Chief Scientist passes by, offering greetings to the "Second Elder". Changing his tone of voice a bit, the City Administrator calls the Scientist back. He takes the antidote bound for Ian and offers to deliver it himself. Not wishing to offend his superior, the Scientist agrees and hurries off.

The City Administrator reiterates his belief that Ian is only feigning illness. Even without this so-called "antidote", he believes Ian will live and his deception will be proven. He smashes the phial on the ground, spilling out the precious drug.

Susan and the First Elder worry over the delay in the arrival of the antidote. They expected it some time ago. Ian is stoic, but it is clear that his strength is fading fast. Susan decides to go to the lab to see what is happening there.

Meanwhile, the Doctor and the Chief Scientist have gone on their "expedition" - to the aqueduct. He is seeking the source of the atropine poison. The entrance is full of large pipes which lead into a very dark cave. The Scientist seems quite afraid of the area and says that all their attempts to light the cave have failed. No Sensorite will go into the darkness. The Doctor fears that their neglect may have resulted in the poisoned water and produces the torch he plans to light his way. He is going in. The Scientist tries to dissuade the Doctor, telling him that they have heard the roars of terrible monsters in the cave. They have never been seen, but the noise was deafening.

The Doctor dismisses the Scientist, assuring him that he will be perfectly all right. The frightened Sensorite cannot leave the area fast enough. The Doctor finds this all very curious... and very convenient. Noise and darkness are the two things the Sensorites dislike, and these things have kept them from the aqueduct for some time. And now the water is poisoned. There is more to this than meets the eye. The Doctor heads into the cave.

Susan has succeeded in getting more of the antidote and finishes administering it to Ian. Neither she nor the First Elder can understand why the first batch never arrived. A grateful Ian sinks back to rest and recuperate.

The Scientist arrives and reports his trip to the aqueduct with the Doctor. The First Elder is aghast that he was left alone down there, even after he was told of the monsters. The First Elder is reluctant to send any Sensorites down, knowing the horrors that have been reported. Ian and Susan are appalled at the First Elder's seeming cowardice and decide they must go down to try and rescue the Doctor. Ian is still quite ill, but he knows he must go. Supported by Susan, and led by the Scientist, they head off to the aqueduct.

The First Elder is impressed at their courage and dedication to one another. He realises that he has misjudged the humans, as has the Second Elder, and he sends a telepathic message to the Second Elder to let him know what has happened.

In the disintegrator room, the Second Elder receives the transmission but cannot respond because the City Administrator has taken his transmitter disk. The City Administrator realises he is hearing a message and forces him to reveal what has been sent. The City Administrator scoffs at the First Elder's assessment of the "noble" humans and his hatred for them grows. However, he also learns about their trip into the aqueduct and he is overjoyed. His work will be done for him: all three of them approach their deaths in the monster-infested caves.

The Doctor's quest is completed in short order. In the darkened aqueduct, he finds deadly nightshade growing right near the intake valve. That is the source of the poison. Susan, Ian, and the Scientist arrive at the entry to the aqueduct. Again, the Scientist is quick to leave. Susan and Ian must go on alone.

Ian and Susan hear the roar as well, followed by the Doctor's shouts of defiance which cut off abruptly. They rush forward and find the Doctor, unconscious in the darkness, his torch extinguished. The back of his coat is torn to shreds, but he is unharmed. Ian lifts him to his feet while Susan collects his sample kit. She notices a piece of equipment attached to a nearby pipe, but is unsure as to its function. With the creature's roar moving further away, Ian figures it is safe to move. They do so hurriedly.

In the laboratory, Carol learns from the First Elder of the hopeless situation facing the Doctor, Ian, and Susan. With the noise and the darkness in the aqueduct, he dares not send his warriors to help them - they would surely be more of a hindrance. They are up against fearsome monsters and cannot hope to prevail. Carol succumbs to despair, but the First Elder reminds her of the rapid progress John is making in his recovery. The final treatment is to be given today and he suggests that Carol rejoice over the friend who is being restored to her. But she cannot help thinking of the others who are gone.

Outside the aqueduct, the Doctor, Ian, and Susan are counting their blessings. The Doctor's coat is ruined, but he is fine and now fully conscious. The creature knocked him unconscious and had him at a complete disadvantage, so it seems strange that it only shredded his coat. Due to the darkness, the Doctor was unable to see what struck him. He chalks his survival up to the timely arrival of Susan and Ian. He learns of the missing antidote that was meant for Ian and becomes very agitated indeed. It appears that there is someone among the Sensorites who bears them ill will. He believes that the poisoned water and the monsters in the aqueduct are connected somehow and he appears confident that he can solve that problem. But he fears that a treacherous Sensorite is a much greater danger. With Ian and Susan supporting him, the Doctor heads back to the city to try and suss out the evil Sensorite.

They do not know that they have been watched all along by the Engineer.

John is being prepared for his final treatment, still mumbling about treachery, secret plots, and warnings. Carol pays close attention, but the Scientist grows tired of this insinuation. Sensorite society is based on trust and equality - secret plots are unheard of and unnecessary. Carol believes there are always individuals who want more than others, but the Scientist dismisses this as a human attribute unknown on the Sense-Sphere.

After a moment to calm down, the Chief Scientist explains to Carol what is being done to John. He says the brain is full of "compartments" in charge of different brain functions. When individual functions are "active", a barrier or veil is lifted on the relevant compartment. It is open. The fear centres of John's brain have become permanently active due to the early Sensorite influence on his mind. The veil will not lower itself, thus he is always fearful and confused. This treatment will allow the veil to close and then function normally once again, opening and closing as needed. Carol likens the veils to her eyelids, not realising that Sensorites do not have them. The Scientist relates this fact with some sadness: "To see all the time is not a good thing".

The treatment is about to begin. John still wishes to warn Carol of the plots he's heard, but he cannot remember clearly. He does recall Carol's name, though, a small triumph in itself.

In the disintegrator room, the Engineer relates what he saw and heard in the aqueduct. The City Administrator is clearly feeling some heat now. The Doctor now knows that someone is against him. Once John is cured, he can speak against him as well. The City Administrator's paranoia grows and he is further angered by the Second Elder's admonitions. He feels it is up to him to destroy these humans that the First and Second Elders have allowed to undermine the Sensorite nation.

The Second Elder is tired of being held captive and tired of all the empty ranting. He challenges the City Administrator to follow through on his threats, hoping that he will be careless and get caught. The plan works, but the City Administrator needs the Second Elder's cooperation to advance his plan. Fearing for his family group, the Second Elder agrees to summon the Chief of Warriors to the palace courtyard with the firing key of the disintegrator. The Warrior hears and agrees. But the City Administrator, still wearing the Second Elder's sash, will be the one to keep the meeting. While the Second Elder predicts doom and disaster for the Sensorites should the City Administrator succeed in his plans, the City Administrator heads for the palace.

The Doctor, Ian, and Susan reach the courtyard just as the meeting is about to take place. Ian, tired from illness and exertion, must rest. They all watch as the firing key changes hands, although they do not know what it is. Susan mistakes the City Administrator for the Second Elder due to his sash and the Doctor goes off to have a word with him about the aqueduct. Meanwhile, Ian and Susan lament Barbara's absence. They resolve to ask the First Elder to allow her down from the spaceship.

The Doctor returns, reporting that the "Second Elder" ran away from him. It is inexplicable. The three of them continue on to the First Elder's rooms.

The City Administrator returns with the firing key, ready to use the disintegrator to finish off his human enemies. However, the Second Elder breaks free of the Engineer, grabs the key, and bends it until it is doubled over. There is a scuffle and the Engineer strikes the Second Elder. Hard. The City Administrator is angry over the irreparable damage to the key; the First Elder himself holds the only other one. But the Engineer presents a worse problem - the Second Elder is dead! The Engineer is in a panic and wishes to flee the city, but the devious City Administrator has a plan to turn this to their advantage. And it involves the Doctor.

The First Elder has heard all that the Doctor's discovered and adds another piece to the puzzle: the "Second Elder" is the one who waylaid the antidote for Ian. The First Elder has been unable to contact him since and can think of no reason why he was in the courtyard and why he would run from the Doctor. Despite all this, the First Elder refuses to believe his adviser has done anything wrong. He trusts the Second Elder absolutely and knows that there is no hard evidence to undermine that trust.

A servant brings a present for the Doctor - a fabulous new cloak. The Doctor models it proudly, certain his old friend Beau Brummel would approve. But this merriment is short-lived. The City Administrator - once again wearing his own collar of office - enters with the Engineer and the Chief of Warriors. The Engineer reports that the Second Elder has been killed, beaten by the Doctor in the courtyard. Unwittingly, the Chief of Warriors corroborates this lie, saying he saw the Doctor run after the "Second Elder" after their meeting. The Doctor denies the charge, but the Engineer adds more detail. The Doctor wrestled for possession of the firing key - which was bent in the struggle - then took an object from his coat pocket and struck the Second Elder, killing him. The City Administrator produces the bent firing key as proof.

The First Elder demands a response from the Doctor, but it is Ian who saves the day. He gets the Engineer to reiterate his description of the Doctor's coat and then reveals that the Doctor's ruined coat is still in the aqueduct. The Doctor displays his cloak and the rattled Engineer changes his description. But the First Elder tells him the cloak is brand new, a gift from him. The Engineer is dragged off, the City Administrator planning to interview him personally very soon.

The First Elder is distressed by this fabrication and by the apparent actions of the "Second Elder", but he does not blame the City Administrator for believing "the Engineer's story". The City Administrator says the Second Elder always disliked the humans and was probably planning to use the disintegrator against them. That also ties in with his hijacking of the antidote. The City Administrator's plan has worked, despite the mistake, and all suspicion falls on the Second Elder.

The First Elder turns his mind to finding a replacement for the Second Elder. The Doctor, Ian, and Susan are fooled into believing that the City Administrator is the prime candidate and they hope to gain an ally by putting his name forward. The First Elder accepts the nomination and offers the sash to the City Administrator. He accepts with an abundance of humility. The humans go off to check on John while the two Elders prepare for a consultation. Ian takes a moment to congratulate the new Second Elder and receives a snide rebuff for his trouble.

In the laboratory, John's final treatment is over. Carol watches excitedly as John comes round. It becomes clear that he is fully restored to normal and Carol cries in relief and happiness. John comforts her, glad to be clear-headed once again. However, his memory of events while he was ill is hazy at best. He takes a moment to thank the Scientist for his work as well, teaching him the Earth custom of shaking hands.

The Doctor, Susan, and Ian arrive and join in the happy scene. John remembers Susan - and Barbara - but must be reacquainted with Ian and the Doctor. While the Doctor and Ian have a word with the Chief Scientist, Susan tries to prod John's memory. He can remember overhearing a plot against the humans but cannot identify who was speaking. The new Second Elder breaks rudely into the conversation, implicating his deceased predecessor in John's plot. Badgered by his insistence, everyone agrees this must be so. The Second Elder brusquely tells Susan to have the Doctor report to the First Elder at once.

The Doctor and Ian have discovered a cache of papers belonging to the first crew of astronauts who arrived on the Sense-Sphere. There are the usual family snapshots, but there is also a rough map of the aqueduct. The Doctor is very interested in it and the Scientist offers to provide him with a fully-detailed plan. Susan relays the Second Elder's message to the Doctor, but suddenly stops short when something occurs to her.

Susan prods John's memory again and he remembers the odd collar the scheming Sensorite wore. It was the City Administrator who was plotting, not the Second Elder. If it had been the Second Elder, John would have recognised the same sash on the new Second Elder and accused him. Clearly, they have been duped by a cunning adversary - the new Second Elder. It all makes sense now. And now their adversary has even more power than before - power they helped him obtain.

Using his power, the Second Elder has had the Engineer released from prison. He has a unique task to perform. The Second Elder shows him two of the paddle-like ray devices used as tools and weapons. The Engineer agrees to disable them so they will not function, but will look untouched.

The Doctor and Ian have an audience with the First Elder. Despite their concerns over the Second Elder, they know they cannot simply accuse him of treachery. They must get proof and they believe that proof is in the aqueduct. They get permission from the First Elder, telling him they are going to find the source of the poison which has been deliberately placed to taint the water supply. They also ask for Barbara to be brought down from the spaceship to look after Susan while they're gone. The First Elder agrees to all of this.

In response to a telepathic message, the Chief of Warriors brings two of the ray weapons to the First Elder's chamber. The Doctor and Ian are to be given instructions on how to use them. They will have light and weapons when they return to the aqueduct. No one knows the weapons delivered have been rendered inoperative.

The Second Elder is congratulating himself on his cunning when the Scientist approaches him in the courtyard. The Scientist carries a map of the aqueduct for the Doctor. Once again, the new Second Elder offers to deliver the plans himself, sending the Scientist on his way. He instructs the Engineer to change the lines on the map, rendering it useless. Now the humans will go down into the aqueduct with useless weapons and shall be hopelessly lost as well.

The Doctor and Ian finish their instruction in the weapons - which only stun and paralyse - and are ready to leave. The plans arrive and the Doctor has only a moment to look at them and does not notice the changes that have been made to them. The First Elder tries once again to dissuade them from this foolhardy venture, but the Doctor assures him they will be fine. He does worry about Susan, however, and instructs the First Elder to keep their departure a secret from her as long as he can. Then the Doctor and Ian are off.

The First Elder and the Chief of Warriors speak well of these brave humans. But the First Elder is troubled. It is clear that the humans did not kill the Second Elder. That means that a Sensorite was responsible for this unheard-of act of violence, a thought that chills the First Elder's soul.

In the laboratory, a celebratory feast has been laid out in honour of John's recovery. Carol, Susan, and John await the Doctor and Ian before they begin eating. Susan is concerned over their prolonged absence and Carol volunteers to go to the First Elder's rooms to find them. When she is gone, John speaks wistfully of their future together. He cannot wait to get back to Earth and give up space travel. He wishes only to settle down with Carol as his wife.

In the aqueduct, Ian and the Doctor enter the dark cave, unsure of what awaits them there.

The Second Elder and the Engineer are of course behind Carol's kidnapping. They bring her back to the disintegrator room, shoving her roughly inside. Carol is defiant at first, but the Second Elder wears down her resistance by reminding her that she is alone and that no one knows she is missing. He could easily kill her without anyone knowing, and he will do so unless she does as he instructs. With no other choice, Carol agrees to write a note to John telling him she has gone up to the spaceship. With Carol under his control, the Second Elder envisions success for all his plans. He has apparently seen beyond the simple elimination of the humans and now envisions himself taking over for the weak and stupid First Elder. He promises the Engineer a high office in exchange for his loyalty.

A short time later, Carol's note is in the hands of the newly-arrived Barbara. She has joined Susan and John and none of them has been fooled by the note. Clearly, whoever forced her to write it was unaware that Barbara was coming down and would have known if someone else was coming up. Carol has been kidnapped. John cannot see why it was done, since she and the others have done so much to help the Sensorites, but Barbara can clearly see the machinations of a power grab above and beyond paranoia.

The First Elder greets Barbara warmly, but she is too concerned over the situation to waste time on pleasantries. The absence of the Doctor and Ian is foremost on her mind. Per his agreement with the Doctor, he refuses to reveal their whereabouts, but says that they are safe.

Barbara turns her attention to Carol's note. The First Elder says she could not have left the planet without his order and he gave no such order. To Carol's friends, this proves that she was forced to write the note as a decoy and is still here on the Sense-Sphere, but the First Elder refuses to believe that any Sensorite could be capable of kidnapping. But Barbara and the others are convinced; and convinced that she is being held nearby as the ink on the note wasn't yet dry when the received it in the courtyard. The First Elder says there is only the palace and the disintegrator room nearby. Neither is a likely hiding place to him.

The First Elder regrets that he cannot help them solve Carol's disappearance, but he tries to set their minds at ease by revealing that the Doctor and Ian are in the aqueduct. Despite his assertion that they have light, a map, and weapons, their friends are very worried to know the truth.

The Doctor and Ian have gone some ways into the aqueduct before realising that their weapons are useless. At the same time, the Doctor notices the altered lines on the map. They are deep in the caves now with no map and no protection. They could probably find their way out in time, but they have no food and only poisoned water. Things look grim.

A scared and angry Carol demands food and water of her guard, the Engineer, but he refuses. He thinks humans weak and worthless and taunts Carol over the pointlessness of her whole race. As he rants, John's face appears at the door. He was so worried about Carol he decided to check the disintegrator room despite the First Elder's comments. Just as John gets fully inside, the Engineer sees him. He grabs a wired electrical prod from the disintegrator's control console and threatens to electrocute Carol if John comes one step nearer. He refuses to be beaten by a human. Suddenly Carol grabs the wire and yanks it out at the base, rendering his weapon useless. John shouts at the Engineer to stun him. The Chief of Warriors, alerted earlier by John, arrives to take the Engineer into custody again while John comforts Carol.

The First Elder discusses this grave matter with the Second Elder a little later, while they wait for the Engineer's questioning to be completed. The Second Elder is alarmed to hear that the First Elder believes the Engineer had an accomplice. He is feeling the heat again.

Susan and Barbara enter. They have been present at the questioning and are reporting to the First Elder. The Engineer has not named his accomplice - much to the Second Elder's relief - but he has revealed the tampering he did to the guns and map given to the Doctor and Ian. This treachery is too much for the First Elder and he swears that the Engineer will die for his actions. But that does not help the Doctor and Ian.

Barbara is not content to sit and fret about her friends. She asks for an authentic map of the aqueduct and the First Elder's help in a rescue plan. The Second Elder tries again to raise suspicion and doubt in the First Elder's mind, but this time he will not be swayed. In repayment for all the Doctor has done for the Sensorites, he promises all the help he can give.

In the aqueduct, the Doctor and Ian move cautiously. They have heard distant roars, but have seen nothing of the creatures. They seem to be keeping their distance. Moving into a new section of pipeline, Ian sees a brief movement up ahead. Using the rolled-up map as a decoy, Ian elicits a direct attack.

There is a brief struggle in the gloom, but no savage roars or flashing claws. Ian loses and returns to the Doctor. His assailant wasn't a creature but a man. A human being! A piece of his clothing came off in the struggle - a shoulder flash bearing part of the word "engineer". The Doctor believes the man is a survivor from the Earth ship that exploded 10 years ago. He has had this theory brewing for some time and now he has proof. He is triumphal. This man - and likely others - have been hiding down here for 10 years, deliberately poisoning the water and fabricating monsters using a speaker system to keep the Sensorites away. (Unbeknownst to them both, Susan discovered one of the speakers near where the Doctor was attacked.) Ian and the Doctor must go and find the man to discover why.

In the palace, Susan and Barbara are shown a scale model of the aqueduct. With it, they hope to be able to help the Doctor and Ian out. Barbara asks for the use of a mind transmitter disk and the First Elder agrees. She and John will go down to the aqueduct to find the others. Susan, whose mind seems to be able to speak telepathically here on the Sense-Sphere, will guide them using the model and her thoughts.

Barbara makes a quick test and finds that she is able to send her thoughts as long as she concentrates. The First Elder warns her she must not let her mind wander while using the disk. Before she goes, Barbara asks the First Elder to put a warrior on guard to protect Susan. One that he trusts. The problem is, he trusts all Sensorites. Barbara heads off to the aqueduct.

The First Elder is impressed with Barbara's intelligence and determination and he tells Susan so. She turns this comment into a question. Why, she asks, does the First Elder trust all Sensorites? Trust must be earned, not given blindly. The First Elder reminds her that their whole civilisation is built on mutual trust, although her words give him much to think about in that regard. He marvels at the wisdom of Earth people, but Susan tells him she and her grandfather are not from Earth. Their planet is quite like Earth, but the night sky is burnt orange and the leaves on the trees are bright silver. She clearly loves her home and hopes to return someday, although she also has a wanderlust that urges her to travel. The First Elder agrees to let them all go when this is over. He only hopes they are all still alive to go.

The Doctor and Ian have searched the aqueduct for some time, the Doctor marking their way with chalk so that they don't go in circles. Ian is tense, but the Doctor couldn't be happier pursuing clues and trying to solve a mystery. While he goes on about it, he and Ian are caught in a trap. Two men appear, one at either end of the tunnel, both carrying sharpened sticks as weapons. The men are shaggy and dishevelled with torn clothes and long matted beards. There is more than a hint of madness in their eyes. They advance slowly on the Doctor and Ian, hemming them in.

The men finally speak, seemingly glad to see the Doctor and Ian despite their obvious fear. They wish to know if all the Sensorites are dead and the "war" is over. Ian says they are, realising it is what they desperately want to hear. Despite this, the men are still nervous and skittery. They refuse an attempt to take them to the surface. Instead, Ian and the Doctor will be taken to see their Commander.

John and Barbara reach the start of the aqueduct and prepare to go in. Barbara requires a little fine-tuning in her telepathic technique before Susan can hear her clearly. With Carol's help, Susan instructs them to enter and bear right. They head inside.

The astronauts are very concerned that the Doctor and Ian might not be alone down here. The Doctor assures them there is no one else. Little do they know that Barbara and John are not far behind.

Barbara finds the Doctor's useless map where it was left after the struggle. They are glad to have some sign of the Doctor and Ian's whereabouts. They move on, taking the map with them as proof that it was tampered with.

Elsewhere in the aqueduct, Ian and the Doctor continue on their long march to the astronauts' HQ. The Doctor has a moment to scrawl a mark on the pipe before moving on. He is still marking their route.

Susan monitors Barbara's progress, but they've still had no luck in finding the Doctor or Ian. The First Elder suggests they may be chasing the creatures, but Susan has a feeling they've been captured.

At last, the Doctor and Ian arrive at the HQ, a dead-end section of cave and pipe made up as a makeshift bunker. There they meet the Commander, a sad parody of a military leader. He is as dirty and bedraggled as the others - Number 1 and Number 2 - but he seems even more mad. He instructs Number 2 to go on "ammunition detail", that is putting poison in the water pipes. New supplies are to be placed in pipe number seven this time. Again, the Doctor's theory has been proven correct. He then sends Number 1 off to prepare a lecture for Number 2 on the necessity of keeping his uniform in good condition...despite the fact that all of their uniforms are filthy rags at this point. He assures the Doctor, with a mad glimmer in his eyes, that morale here is very high despite the trying times.

The Doctor and Ian realise these men believe they are at war with the Sensorites. They have been waging a guerrilla war for 10 years and they are weary; they want very much for it to be over. They treat Ian and the Doctor - the first humans they've seen in many years - as comrades and seem to accept what they say as truth. The Doctor tells the Commander that they have won the war. The Sensorites are all dead thanks to their poison and the planet is theirs. The Commander is pleased and relieved, of course, but still wants to tell war stories. He explains that two of his men deserted and tried to steal their spaceship to return to Earth. He was forced to blow it up to stop them. Now the war is over, he can claim the rich molybdenum deposits and buy a new ship.

However, when Ian indicates that he knows of the mineral wealth here, the Commander becomes paranoid. He and his men have won the "war" and the spoils belong to them alone. He becomes agitated, pacing and questioning the Doctor and Ian again as to whether they are alone here. He rants about the power of his men and his "organisation", and when Number 1 returns with news of other intruders, he immediately believes the Doctor and Ian are Sensorite spies. The war is not over at all and he orders an immediate court martial for them.

John and Barbara are brought in and there is a brief reunion. The Doctor tells the Commander that the newcomers are part of a welcome committee to bring them back to the surface and to celebrate their victory over the Sensorites. Barbara plays along gamely. Still suspicious, still mad, but ultimately just tired, the Commander accepts this story. He wants the "war" to be over so badly. He agrees to go to the surface with his men. They leave immediately.

At the entrance to the aqueduct, the Chief of Warriors awaits the humans' exit, alerted by Barbara and Susan. Number 1 and Number 2 lead the way out, giving up easily when confronted by the armed Sensorite. They drop their weapons wearily and without a fight. They are led away. But the Commander is angered at this seeming betrayal and tries to fight back. The Chief of Warriors stuns him and he collapses.

The Doctor has pity for the astronauts, but the Chief of Warriors harbours much anger over the deaths they have caused his people. He restrained himself from killing the Commander, even though he wanted to. The Doctor applauds this, seeing great promise for the future of the Sensorites.

Some time later, back in the First Elder's palace, things are getting back to normal for the first time in a long time. Maitland has been given permission to return the insane astronauts to Earth for treatment. It is clear that they had all been damaged by the Sensorites' telepathic powers and driven mad. They played a game of war with deadly consequences. The First Elder tells Ian and Barbara that the Second Elder has been found guilty of his crimes and has been banished to the outer wastes. He will not return.

It is now time for them all to return to their ships. The First Elder reports that the TARDIS lock has already been replaced and they are free to go. He sends them off with the gratitude of the Sensorite nation.

Later, in the TARDIS, the Doctor and Susan await Ian and Barbara, who are saying a final goodbye to John and Carol. Susan is saddened to learn that her telepathic abilities will likely diminish once she leaves the Sense-Sphere. It was a feature of the planet that allowed her to communicate so easily. The Doctor encourages her, promising to work on her latent telepathic powers once they return to their home planet. When that will be, though, is a mystery. The TARDIS seems to be an aimless thing somehow. Susan seems to appreciate this, but part of her still wishes to find some place to settle down and belong.