On an Earth colony in the far future, life is a ball... literally. A crowd is gathered at the entrance listening to a jolly band and watching a drum majorette. The man leading the band is called Barney. He receives congratulations from the Pilot of the colony on the quality of his band. He and the group also receive trite aphorisms which pass for encouragement. Clearly all is well in this happy place.

However, there is at least one person unhappy with the colony. A wild-eyed man called Medok bursts onto the scene, racing for the exit just ahead of some pursuing policemen. Medok pulls up short when he sees the Pilot. The Pilot is clearly aware of Medok's "problems" and tries to tell him how foolish he is being. What exactly he's doing that is foolish (escaping, rebelling, running) is not clear just yet. But it is clear that Medok is not like the other happy citizens. Medok stops only for a moment and then pushes past the Pilot and escapes outside.

The chief of police, a large man called Ola, is furious at this and orders his men to pursue Medok. Ola explains to the Pilot that Medok refused "treatment" in front of the guards and so he dismissed the guards to "deal with him" alone. Medok took advantage of this and escaped. He might be small but he is clearly very determined. Both Ola and the Pilot find this little man extremely dangerous for some reason. He must be recaptured.

Things in this colony are certainly not as benign as they first appear.

Medok is not far ahead of the pursuing guards and so decides to try and hide. He finds a niche in some rocks and hunkers down, waiting for the guards to pass. They do so, but just as he starts to get up, Medok hears a strange noise. With engines grinding, a strange blue box fades into sight before him. He watches in amazement.

Ben is first out of the TARDIS, followed by the others. All of them seem jumpy and nervous because of the hideous claw they saw on the time scanner. They expect to run into the creature in question at any moment. Jamie has even armed himself with a large stick, just in case, although the Doctor surmises these creatures are very large and would not be deterred much by such a weapon.

However, Medok is alarmed by the weapon and decides he must do something. He jumps from hiding and attacks Jamie. Ben, Polly and the Doctor join the fray and pull Medok off, all of them surprised at the ferocious strength of the disheveled little man. Ola and the guards hear the commotion and approach, pleased to find their quarry already subdued. Ola thanks the strangers for their help, explaining that Medok is an escaped "patient" who they fear is beyond their help. He makes certain to note that he is "one of their last" patients.

Ola has his guards tie Medok up, becoming very vicious in making sure the ropes are tight. As the men get on with it, Ola turns to the strangers, speaking sweetly now about what a responsible job it is to be chief of police. He seems to have two extreme sides to his nature. He invites the strangers to accompany him back to the colony. The Doctor agrees, although he seems concerned about Medok. He admonishes the guards to treat their prisoner a bit more gently.

Once back inside, Ola introduces the strangers to the Pilot over the sounds of a cheerful pre-recorded jingle issuing loudly from a speaker. He, too, is sweetness and light as he welcomes his guests. Ola is sent off to "take care of poor Medok". The Doctor seems uninterested in the niceties and finds the whole thing a bit fake, especially all the music which regulates the day. He leads the group through to the refreshing department. Jamie, too, is unnerved by all this positivity. He tells Polly the colonists are "a weird sort of folk" that he doesn't entirely trust.

The refreshing department is a combination gymnasium and health spa, complete with attendants. Before they can explore, however, the Pilot is given a message: Control wishes to welcome the guests. Clearly this is someone of higher rank than the Pilot and he responds immediately, switching on a nearby television screen. The photographic image of a handsome, virile-looking man comes up. The Controller. Polly is impressed. A jolly voice, full of energy and authority, bursts from the speaker welcoming the strangers to the colony. He is grateful for their help in capturing Medok and the colony should do everything it can to make them happy and contented during their stay. The message ends with a bouncy exhortation to "get back to work".

Polly is pleased by all this to-do, but the Doctor seems very quiet. The Pilot turns the visitors over to Barney, who is the supervisor of the refreshing department. He, too, is unflaggingly cheerful as he introduces them to all of the delights available for them here: steam baths, beauty treatments, massage, etc. Polly asks for a shampoo and she is lead away. Ben and Jamie are led to two other cubicles by the lovely young female assistants. Ben in particular is pleased at the treatment.

Polly returns after a bit, finished with her hair very quickly, and Barney compliments her on her new hairdo. She blushes, but he assures her it is all part of the service, putting an odd emphasis on the last word. Only the Doctor notices this. Jamie is having a hard time fending off the attentions of the lovely ladies; no one moves to help him.

The Doctor is placed in a machine called a clothes reviver which envelops him completely. Barney activates it against the Doctor's wishes and he emerges a moment later immaculate, his clothes neat and tidy. Even his suede shoes sport a shine! The Doctor responds by jumping onto the rough and tumble machine despite the protests of Ben, Polly, and Barney. He emerges more rumpled than ever, feeling ever so much better.

Jamie has no such luck. He has been turned into a veritable strutting peacock by the attendants, much to Polly's approval.

Elsewhere, Ola and his guards lead Medok back to a cell to await transportation to the hospital for correction. A crowd watches at a distance as he is led, shouting, down a corridor. Ola tells the people that Medok is violent and suffering from delusions, but Medok shouts back that everyone else is deluded. They are closing their eyes to what is really going on in the colony. From the crowd, a man steps forward. He is called Questa, an old fried of Medok's. Questa cannot believe what has become of his friend, once as cheerful as the rest of the citizens. It is clear that cheerfulness is a highly-valued commodity here, and that lack of it is deemed dangerous.

Questa is allowed to speak to Medok and asks him about his "seeing things". However, Medok turns the tables on Questa, insisting that he's only "seeing things" which are really there. Horrible creatures infesting the colony at night.

The voice of Control issues from a speaker, announcing the end of shift rest. The colony needs its workers and they should go back to work at once. He also announces a welcome reception for the strangers tonight, saying it will be "fun for all". The benign words sound like an order. Despite all this happiness and joy, Medok begins raving again, telling everyone who can hear him that "the party" will be over once these creatures begin to crawl all over them.

The Doctor arrives to hear this last part of Medok's declaration and becomes worried. He is connecting what Medok has described to the creature whose claw he glimpsed on the time scanner and he wants more information. However, Ola will allow no one else to speak to the prisoner and shoves him on toward the nearby cell. The Doctor is left alone in the corridor, trying to decide what to do. He is startled by a young woman, named Chicki, who asks if he needs any help. The Doctor brushes her off and she goes away. As soon as she is out of sight, he begins picking the lock that leads to Medok's cell.

Before long, the Doctor is in the cell with Medok, who is frightened of this stranger even though he promises to set him free. The Doctor does so and then asks questions about the creatures that Medok has seen. The man is very frightened and jumpy. At a slight noise, he clams up and believes that Ola and the Pilot are here to get him. In an unexpectedly sudden move, Medok shoves the Doctor out of his way and escapes through the open door. He is determined not to be taken to the hospital again and races to escape from the colony. An alarm sounds immediately.

The Doctor chases after Medok but is stopped by Ola and the Pilot. They are furious at the escape and blame the Doctor. Ola proclaims the Doctor a criminal himself and says that he should be forced to work in the pit. Ben, Polly, and Jamie arrive to stick up for the Doctor, claiming that he did not know the law. The Pilot agrees and thinks Ola should concern himself only with stopping Medok; he himself will decide what to do with the Doctor. The Doctor presses his point, insisting that Ola hurry. Medok can run, not crawl over the ground... As expected, Ola rises to this bait and becomes angry, saying the Doctor could be sent to the hospital for corrections for spreading such malicious rumours.

The Pilot intervenes in this fracas to calm the mood, saying calmly and smoothly that the Doctor should be given a better picture of Medok's erratic behaviour and of life in the colony before he can truly understand what they are trying to do. Ola's protests are silenced and he is asked to take the strangers to the labour centre for a bit of edification. The Doctor is pleased that civility reigns.

The group must go outside and cross a stretch of open ground to reach the labour centre and the Doctor pauses at a building site along the way, thinking he has seen or heard something. He shrugs it off and moves on. The voice of Control even reaches outdoors, announcing that even though their "unfortunate companion" Medok has escaped, no one should be alarmed. Two patrols are out searching for him and he is sure to be caught before dark. They can all return to their work and play "with fresh heart and renewed energy". Ben is awed by the falseness of it all, even more so when a little jingle about the fun of working issues from the speakers. He hasn't known a lot of people that happy about their jobs.

There are a lot of workers passing through the labour centre as it is shift change. The strangers are greeted by Officia, the chief of operations. Jamie is a bit perplexed though as he cannot ascertain what labour is actually done here. Officia explains that they tap and refine gas, although he is not specific on the gas' use. He only says it is absolutely essential to the functioning of the colony.

Their conversation is interrupted by an alarm and the arrival of two miners staggering in from the pit. There has clearly been some kind of accident. The miners are given oxygen to combat what Officia calls "gas sickness", an unfortunate fact of life in their business. It seems that this "absolutely essential" gas is poisonous to people. Polly turns to get the Doctor's response to all this and is surprised to find that he is no longer with them.

He has returned, unseen, to the building site they passed on their way to the labour centre. There he finds Medok in hiding from the guards. He is frightened and angry when the Doctor finds him and is very hostile. The Doctor tries to convince him that he will not give Medok away to the guards and that he is only interested in finding out more about these creatures. Finally, Medok relents and tells what little he knows.

The creatures are active only at night and are difficult to describe. He finally calls them "giant insects" with big claws. Others have seen them, but they are either locked up in the hospital for correction or they have been brainwashed by Control into forgetting. The Doctor himself could be put there for helping Medok.

In the distance, Ben and Jamie call for the Doctor. They are approaching the building site looking for him. They find him walking casually about in the gathering night, tootling on his recorder and acting innocent. Ben, Jamie, and Polly take him at his word, but the guards with them are not as forgiving. All citizens are confined to rest cubicles at night by order of Control as it is dangerous to wander around outside after dark. Perfectly timed, the voice of Control issues from the speakers announcing curfew. Night shift workers will remain at their posts and the visitors shall retire to the refreshing department for the night. His voice sounds innocent, but the guards insist on using the word "confined".

In the refreshing department, Barney directs the Doctor and his friends to their rooms and retires. However, as soon as he is gone, the Doctor heads outdoors again. The voice of Control addresses its citizens, warning them to stay indoors due to the dangerous man on the loose. The guards will deal with him and everyone else should enjoy a happy sleep time.

The Doctor heads immediately for the building site to meet up with Medok. He must avoid Ola and his patrol as he goes. Medok is still very skittish and wary. The Doctor cannot seem to make him understand that he is here to help. Suddenly they hear Ola's voice, very near. The patrol is going to search the building site and Medok begins to panic. This makes it very difficult for the Doctor to act calmly.

When the guards approach the front of the house, the Doctor directs Medok to the back. He seems to settle down somewhat due to the Doctor's presence. He is glad for the first time to have someone believe him and to have company in his opposition. Together the manage to escape the house from the rear as the guards enter, but they don't get far.

The Doctor watches in awe as the giant creature scuttles back into the darkness. Medok is triumphal as they retreat back into the partially-built house. This is proof that he hasn't been imagining things now and the Doctor can back him up. He is so happy he begins raving, heedless of the guards nearby.

Ola and the guards find and capture them both. He is unamused and somehow not surprised to find the Doctor mixed up with this dangerous criminal. But Medok believes that with the Doctor's corroboration even the Pilot will have to believe that these creatures exist; he cares little for Ola's threats. For the first time, Medok gives the creatures a name: Macra.

But Ola is not interested in Medok's claims; he is interested in the Doctor. As he suspected earlier, the Doctor has proven himself heedless of the law and Ola is happy to bind him and take him before the Pilot. Medok now regrets that the Doctor's concern for him has led to trouble.

Even at night, the Pilot is hard at work, urging his workers on to greater productivity in the vital gas mines. He is interrupted by the arrival of Ola and his guards. They bring the Doctor before him. The Pilot seems shocked to learn that the Doctor was out in the colony at night against the rules. There was supposed to have been a guard placed on the strangers' quarters. The Doctor does not miss this offhand comment - he is well aware that things here are not as benign as they appear. The Pilot dismisses Ola and addresses the Doctor alone.

The Doctor notes a telescopic device in the office and compliments the Pilot effusively on its advancement and value. This is clearly boot-stuffing and the Pilot knows it. Despite this, the Pilot remains calm... until the Doctor hints that he believes Medok. The Pilot becomes angry, but before he can glean more from the Doctor, Ola interrupts again. Medok has made a statement.

He is brought in and the Doctor is concerned over what he might say - not because Medok is mad, but because he is so agitated he might say anything. But when he enters, Medok is calm. He says that the Doctor was trying to get him to turn himself in and that Ola did not give him a chance to explain that fact. The Pilot immediately becomes his sweet self again and thanks the Doctor for his help. The Doctor obliquely thanks Medok for his act of self-sacrifice as he's led away.

The Doctor learns that Medok will be returned to the hospital for correction and given another course of "treatment". The Pilot is confident that this will change him back into a cheerful and obedient citizen, like all the others. It was the will of the colony's founders that everyone should follow the virtues of healthy happiness. That ideal has continued through the centuries, but the Doctor is horrified to hear the Pilot extol the necessity of force to make people conform to the ideal. The Pilot cannot seem to see the complete corruption of the ideal contained in his words.

Without another word, the Doctor is sent back to his room. When he is gone, a screen comes to life with the still photograph of the handsome young Controller and the usual fanfare. The voice of Control announces that the strangers are to be given the "advantage" of high powered adaption. Mind control will be used to make them think like all others - all criticism of the colony must be eliminated.

The Pilot relays these instructions and preparations are made. The voice of Control continues its rantings, however, calling this an emergency. All in the colony must obey Control. There are no such things as Macra! No such things as Macra!! It sounds like an insane mantra of fear and denial.

Later that night, Ben and Jamie are the first to be subjected to the adaption process. In their rest cubicle, a soothing voice calls out to them, urging them to relax. It tells them that everything in the colony is good and beautiful. They must obey without question and work hard without complaint. Ben sleeps soundly through all of this, but Jamie tosses restlessly. He can hear the voice clearly and it unnerves him. He sits up, approaching Ben.

Ben has been sound asleep the whole time, listening subconsciously to the voice. Jamie tries to tell him of the voice and the evil nature of it but Ben just turns over to go back to sleep. He says that this is a good place and that they should rest now. They've got a hard day's work ahead of them tomorrow. Jamie is taken aback; this sounds very like the words that the voice spoke earlier. In fact, Ben says that they should give something back to the colony for the food and shelter they are providing. He thinks it won't be too bad to work here - completely the opposite of what he said earlier in the day.

Jamie realises with some fear that Ben is not himself, but suddenly a thin gas begins to fill the room and Jamie falls back on his bunk. The voice returns, urging them both to breathe in the sleeping gas and listen to its words. In the morning, they will both obey...

In Polly's cubicle, she is fast asleep, but she is not alone. The Doctor sneaks in, moving quietly. He examines the wall above her bed and finally finds what he's looking for - a thin strand of wire embedded in the material of the wall. Using a strange instrument from his pocket, he fuses the wire, causing it to short and sizzle. Polly awakes with a start and the Doctor tells her he's been "scotching a rumour". He is acting strangely and doesn't make much sense as he tries to explain the wire and how it is used to pass on instructions to her subconscious while she sleeps. He calls it "bad advice" and urges her to forget all about it, and about any instructions she may have been given. She seems to be OK and the Doctor is satisfied. However, he suddenly realises that he's spent too much time explaining and has forgotten all about Ben and Jamie.

The Doctor races to their cubicle, not bothering to be quiet about it. Ben awakes to find the Doctor hovering over his bunk examining the wall. At first Ben is calm and pleasant, glad to be awake and raring to go. However, once he realises what the Doctor is up to, he becomes increasingly angry. He accuses the Doctor of breaking the colony's law, a law of which he was ignorant until last night. The Doctor manages to short out the wire and Ben promises the Doctor will be thrown in jail for the vandalism.

Polly stands up for the Doctor, not understanding at all why Ben is behaving like this. The argument wakes Jamie, who admits he hasn't slept much at all tonight. The Doctor seems pleased and fuses the adaption wire above his bed as well.

This sets Ben off again and he vows to turn the Doctor in himself. He is now fully obedient to Control and the law of the colony. His friends are shocked to hear this from him. The Doctor is saddened that he was too late to help Ben and Polly and Jamie both try to restrain him from leaving the room. Ben fights back and the Doctor is distressed to see his companions fighting among themselves. He urges Polly and Jamie to let him go, knowing that this violence will get them nowhere. Ben gets loose and goes to fetch a guard.

Polly is distraught and Jamie is angry. Both wish the Doctor to run, but he must stay. He cannot leave Ben, despite what he's done. He knows that Ben is not in control of his actions and he is desperate to help him.

Ben returns with Ola and some guards. The chief of police is very pleased at this turn of events - he is very keen to chuck the Doctor in jail or in the hospital for correction. Ben's evidence will make this possible. Ben stands by like an obedient toady as the Doctor is led away. Angry, Jamie tries to get at Ben for this betrayal but Ola holds him back. Undaunted, Ben shouts out that the Doctor should be taken to the hospital as he needs correction more than Medok does.

In the hospital, Medok undergoes hardcore corrective treatment. He is strapped down as lights and voices assail him. The voices urge him to cooperate and to forget that he has seen Macra. They repeat endlessly and he struggles to block them out. It is a much more aggressive form of adaption, but he is resisting it. The Pilot looks on as the treatment progresses, saddened that Medok seems a hopeless case. He'll never be happy like the rest of the citizens and should probably be sent to the pits for life.

The Pilot receives a message about the Doctor's arrest and leaves the hospital. Medok continues to struggle with every fibre of his being.

In the rest cubicle, Polly fumes at Ben's treatment of the Doctor. Jamie has already gone off after him and Polly decides to go too. She has stayed in hopes of convincing Ben that he was wrong, but she can see that his mind is made up. Ben warns her to stay put, but she runs off, daring him to report her too. She is devastated. Ben follows her, not out of concern but out of duty.

Ben follows her outside but loses her amid the construction site where the Doctor and Medok were hiding. He hunts for her systematically, determined to find her. There is a distinct edge of anger in his voice as he calls for her; he is totally in thrall to Control.

Eventually, he corners her and she cannot escape although she struggles against him. She thinks he must be hypnotised or sleeping and tries to get him to snap out of it, but he is ever the dutiful citizen of the colony. Suddenly Polly stops struggling and screams, her gaze fixed in the distance. She has seen a Macra scuttling about in the dim gloom of early morning. It is huge and hideous, and she recognises the claws from the scanner. It is much more frightening than she even expected.

Ben sees nothing and starts to drag Polly off in the direction of the creature. She becomes more frightened and more determined not to go with him. But she cannot break loose and must resort to pleading with him. Unfortunately, she is proven right as the giant crab-like creature comes into view before them, its eyes glowing in the darkness. Ben can clearly see it, but his conditioned mind fights against what his eyes are registering. He repeats, mantra-like, that there is nothing evil or harmful in the colony. While he is doing this, a giant claw grabs Polly and pulls her from his grasp.

Polly struggles, kicking and screaming against the evil creature, begging Ben to help her. Her screams grow more frantic as Ben stands by like an automaton. It is as if he must process what he sees and is finding it incompatible with his conditioning. But the screams are insistent and they get into his mind. Suddenly Ben snaps into action, grabbing a piece of loose wood and attacking the giant claw. He is acting on pure instinct, overriding the conditioning he has been subjected to. He knows what he is doing, but only on a base level. His friend is in danger and he must act.

It takes quite an effort, but eventually the Macra drops Polly and scuttles off into the darkness. She is free and unharmed but frightened nearly to death, trying to catch her breath between sobs. Suddenly she screams again. Another of the giant creatures is approaching. They are all around now and escape looks doubtful. Still in instinct mode, Ben shoves Polly toward a gap and shouts to her to run for it. He will try to keep them occupied so she can escape. He lashes out with his makeshift weapon.

Polly refuses to leave him and drags him with her. He continues batting at the giant claws with the piece of wood and eventually they are able to slip between the creatures and get free. They run for their lives.

The Doctor and Jamie stand before the Pilot. His patience is wearing very thin and he demands an explanation as to why the Doctor destroyed the nerve circuits. The Doctor is not apologetic and is even proud of his accomplishment. In fact, he notices the same circuit in the Pilot's wall and fuses it as well. He seems concerned that the Pilot has been a victim of this mind-control as well, having assumed that he was behind it. Ola bursts in with Ben and Polly and an urgent report.

Jamie expects that Ben is just going to rat on them again, but it is a shaken Polly who speaks. She explains the attack by the crab-like creatures and how Ben saved her at the risk of his own life. Jamie is surprised at this, thinking Ben is a full-blown traitor.

The Pilot asks Ben to corroborate this story but he is no longer acting on instinct and the conditioning has kicked in again. He doesn't even remember the creatures or the attack; he simply went after the runaway girl and brought her back. Polly is astonished but Ben simply keeps repeating that there are no such things as Macra, more and more insistently.

The Doctor tries again to explain the situation to the confused Polly, but the Pilot steps in. He too believes there is nothing harmful in the colony. Control always acts for the best. The Doctor turns on him, demanding to know who Control is. The still photograph is not the Controller and he goads the Pilot into contacting Control.

He does so, bringing up the still photograph and the disembodied voice. It accuses Polly of hallucinating. Jamie joins in the jeering, trying to force the real Controller into revealing himself. He and the Doctor keep up the barrage until the Pilot relents and nervously agrees to ask for the Controller to be revealed in person. The screen goes blank and the Doctor is certain that they will not produce the real person.

To his surprise, the screen lights up again a moment later. This time, there is a live picture, but it is that of a frail, frightened old man. It is the same man as in the photograph, as he really is. His hair is white, his eyes are sunken, and his expression is vacuous. The Doctor is stunned. Jamie is certain this man is not in control and wishes to hear him speak for himself. Polly joins in the request.

Amazingly, the voice of Control responds and urges the old man to speak. The order is given as if to a dog. The old man responds as if stunned, not sure where to look or what to say. His voice is frail and full of fear. He promises to be obedient to the voice but he pauses for a long time.

Suddenly a giant claw comes into the picture and grabs the old man, dragging him off.

Chaos breaks loose in the Pilot's office as the Doctor, Polly, and Jamie watch the disgusting spectacle. "Control" clearly has nothing to do with the humans on this planet. The Pilot must shout to be heard over the din, condemning the Doctor and his friends to the pits. The guards wrestle with the prisoners to drag them out, but the Doctor and the others cannot take their eyes off the image on the screen.

The Doctor, Jamie, and Polly are hustled from the Pilot's office, condemned to the pits for what they've seen. Ben, loyal servant of the colony, is left with the Pilot as he calls to the supervisor of the work shift. The strangers are to be placed on the Danger Gang, tapping new gas reserves in the most dangerous area of the mines.

The voice of Control rewards the Pilot for his wise action in disciplining the troublesome strangers. He also encourages the Pilot to forget all he has seen. The Pilot goes blank and accepts the order. It is back to business as usual for him: hard work and happiness are all that matter. The Pilot turns to Ben and sends him to the pits as well. He is to serve the colony by spying on his former friends.

Ola escorts the Doctor, Jamie, and Polly to the pithead where Officia is waiting to receive them in the gas centre. As his names implies, Officia is all business and is little interested in the Doctor's observations of the colony's motivational jingles or the mining equipment. He calls up their shift leader.

Jamie is still confused as to the nature of this "mine". It's nothing like the coalworkings he is familiar with. Soon their shift leader appears - Medok! He has been chucked out of the hospital for corrections as a hopeless case and now works on the Danger Gang for the rest of his life...which will likely be a very short time due to the nature of the gas they are tapping. He seems glad at least for sympathetic company.

Officia is ready for them all to go down immediately, but an idea occurs to Medok. He is entitled to a supervisor topside on this shift and he decides to leave the Doctor here. Not understanding at first, the Doctor tries to give Polly the safer job on the surface. But Jamie soon makes him realise that he may be of more value on top... near all the vital machinery. The Doctor understands and agrees to stay behind.

After they are issued with masks, Jamie and Polly follow Officia and Medok down into the pit. The Doctor is given a checklist with which to busy himself. He is quickly scanning the banks of instruments and equipment, already noting some important "adjustments" he could make.

In the meantime, Ben has arrived and is watching him closely from hiding. When the Doctor spots him, he tries to slip back into the shadows but the Doctor draws him out. Ben is following the voices in his head and is being a good little watchdog. All the while, the Doctor speaks plainly to him about his betrayal and the manipulation he is being subjected to. He won't answer questions but the Doctor knows why he is here. He warns Ben that if he plans to spy on the others, he'd better watch out for Jamie. He's not as "tolerant" as the Doctor is.

Down in the pit, Jamie continues to grumble about Polly being forced to work. Officia will say only that this is a punishment for all of them. For Medok's part, he would rather be here than the hospital. His lungs may be in danger due to the toxic gas, but at least his mind is free.

Suddenly an alarm sounds - a big gas strike. The workers must work together to get a massive cable down to the strike area to seal it off. After a long struggle, they succeed. The leak is sealed at last and the alarm stops.

The exhausted workers regroup and the same question occurs to them all. But even Medok can't answer why they risk their lives for this gas. It is deadly to humans and no one knows the purpose for which it is mined.

Back at the gas centre, the Doctor is scrawling calculations on a wall, talking to himself as he goes. He has worked out a formula to do with the gas production and transfer and is very pleased with himself, giving himself 10 out of 10 for the work. The Pilot enters and is stunned to see this top secret formula scrawled on the wall for everyone to see. It is a secret known to only three people in the colony - perfected over a long time - and he finds it highly unlikely that this scruffy vagabond could have worked it out on his own. The Doctor alters his mark and gives himself 11 out of 10!

The Doctor explains - as innocently as he can - that he simply worked the equation backward given the information on the dials and gauges around him. But in the end the Pilot is uninterested in this. He merely wants the equation removed before anyone sees it. The Doctor is forced to scrub the wall clean.

With the excitement of the strike over, Medok, Jamie and Polly are set to work drilling in a new area. The miners are always looking for new gas reserves and the drilling never stops. While they work at drilling a new hole, Jamie notices a brand new, very solid door set into the rock face and asks about it. Medok thinks it might lead toward an old unused shaft but Polly thinks this unlikely. The group is joined by Officia and Ben and they immediately clam up.

Officia is convinced that Medok is using the wrong tools for this particular area and steps in to correct him. Suddenly a burst of gas issues forth and Officia staggers. Ben takes charge while Medok plugs the leak. Strangely, Jamie helps Ben haul Officia to his feet. Ben takes him away for some fresh air while Jamie shows off his prize: he has picked Officia's pocket and removed his keys in the confusion.

Worried that Ben might have seen them, Jamie wishes to act fast. Without consulting Medok, he decides to try the door in the rock wall while Polly keeps watch. Medok's attention is focused on the new drill hole and the dangerous pressure which might be building up behind it. He calls out for help. Polly is torn as to which way to go as Jamie is very close to getting the door open. In the end, she chooses to go help Medok, leaving Jamie to enter and edge down the dark tunnel on his own.

After an agonising moment, an alarm sounds in the pit. Jamie's escape has been noticed. Polly admits it was Jamie and Medok jumps at the chance to follow him through the door. Polly is, unfortunately, left behind.

At the gas centre, Officia has recovered from his exposure to the gas and is now in charge of response to the escape alarm. He is quite worried as no one has ever escaped from the pit before; he will not have a successful escape on his watch either. He orders the guard to find the escapee and shoot him on sight. The Doctor listens in, quite worried over all this talk of shooting. Officia becomes even more alarmed when he realises that his keys are missing and that the escapees may have them. He issues another order for the guards to cover all exits to the old mine shaft. He fears that the guards may have to go on "forbidden ground" in order to find the fugitives.

In the old shaft, Medok edges along the darkness, looking for Jamie. He is calling out but suddenly his voice is cut off. A giant Macra claw closes around him. Further along, Jamie hears his cry and turns toward it.

In the gas centre, Officia is given a response to his request. Control refuses to allow the guards on forbidden ground in their search. Nor are they to cover the exit to the old shaft as Control is using it at the moment. There is no further explanation for this extraordinary order and Officia doesn't seem to need one. Control knows best...despite the fact that Officia himself will be punished if the prisoners manage to escape. The Doctor finds this blindness incredible but Officia will discuss it no further. He goes off to try and find out who it is that has escaped.

However, Ben knows the answer to that question. He tells the Doctor that it was Jamie who escaped; he saw Jamie take Officia's keys. But he did not report it. The Doctor is pleased at this and at the confusion in Ben's mind. He is still aping the words of Control but seems to be exerting his own willpower. The Doctor eggs him on to more resistance but Ben snaps back, heading off to report the stolen keys.

Jamie finds Medok on the cave floor. But he is too late - Medok is dead. Jamie turns to keep moving, but he stops dead when he comes face to face with the giant Macra.

Polly returns to the pithead to be with the Doctor. She is concerned over Jamie's safety, but the Doctor is very optimistic. As long as Control doesn't want the guards in the old shaft, Jamie's got a fighting chance. The Doctor's attention is diverted by a phial that he is busily examining. It is a sample of the gas from the pits and he is trying to analyze it. So far, he's not having much luck. He believes that it holds the key to all that is happening here in the colony if he can only get to the bottom of it. But time has run out as Officia returns.

Ben has taken himself to the Pilot's office to report what has happened but the Pilot is not there and he is becoming more and more agitated. A young woman called Sunna offers to help the anxious-looking Ben. His mind still struggles to free itself from the grip of the voices even as Sunna tells him that the voices are their friends. Ben struggles to remember his own friends...his real friends.

Jamie braces himself to be attacked by the giant creature but it makes no move toward him. He even throws a stone at it with no reaction. He thinks it may be dead. But after a long moment, the creature's eyes open.

Officia is livid at the Doctor and Polly and promises to report them once the emergency is over. As if in response to this, the alarm bell ceases ringing and Officia says the emergency is over. The guards are closing in on Jamie.

In reality, Jamie is merely face to face with the giant Macra, too scared to move.

The voices reassert control over Ben and he tells his story to the Pilot. He was instructed to report to him and not Officia, who still does not know of the stolen keys. The Pilot announces over an intercom that Jamie is in the old shaft and he awaits instructions from Control on what to do. The answer is astonishing: nothing. No one is to go near the old shaft for any reason. Without question, the Pilot obeys.

Officia is waiting for more instructions. Shortly the voice of Control orders gas to be diverted into the old shaft and Officia goes to check the needed machinery. Polly is concerned about Jamie's safety, but the Doctor has another concern. He seems sure that the gas is not intended to poison Jamie as that seems such a waste of a valuable resource. No, there is some other reason for the diversion of gas but he cannot figure out what it is.

In the old shaft, Jamie is still frozen in place staring at the motionless Macra. It is a nerve-wracking game of cat and mouse.

Polly is becoming hysterical, interrupting the Doctor's ability to think. He has figured out that the gas is vital to the Macra but that it only occurs naturally deep down in the planet's tunnels. Since they have come to the surface, they have forced the colonists to bring the gas up to help them live. They are not wasting it by sending it into the old shaft. It is being sent there because there are Macra there!

The Macra facing Jamie begins to lurch slowly forward. Jamie backs away. The creature is moving slowly and sluggishly, as if drugged or asleep.

Officia reports to Control that all is in readiness for the gas transfer and he receives further instructions. The Doctor, forgotten in the chaos, is trying to work out some way to block the gas flow into the old shaft. He is thinking quickly but he may be too late as Officia turns on the pumps.

The gas floods into the old shaft and Jamie begins gasping and coughing. The effect of the gas on the Macra is immediate. It begins to move more surely and quickly and its claws begin to flail. Its forward motion speeds up, moving toward Jamie.

The Doctor thinks he's identified the pipes in question but Polly fears there's now nothing to be done. Officia increases the rate of gas flow, steadily inching toward full pressure.

The Doctor is desperate to do something to help Jamie, certain that he is facing death by Macra in the old shaft. He flits from one dial to another, from one pipe to the next. Officia stands by the pumps, maintaining full gas pressure as ordered. With Polly's help, the Doctor determines which pipe is gas inflow and which is oxygen outflow. To help Jamie, he must switch them. But how?

In the old shaft, Jamie is well and truly trapped between the two Macra. There is nowhere to run. In desperation, he wedges himself into a narrow crevice of work, likely praying to somehow turn invisible.

The Doctor examines the mass of pipes which leads from the gas centre down into the mineworkings, tapping randomly. All the activity at last attracts Officia's attention and he is suspicious. The Doctor resorts to confusion tactics, bamboozling Officia into revealing the location of the controls for the inflow system. With that knowledge, he can save Jamie.

He moves quickly, turning controls left and right, much to Officia's horror. No one has ever broken the colony's rules with such blatant disregard before and he's not exactly sure how to deal with it. The Doctor announces that he has "revolutionised" the entire gas flow of the colony. What he's done is to completely reverse it. Fresh air is now being pumped into the old shaft. An apoplectic Officia can only conclude that the Doctor is made!

Jamie's hiding place has been discovered and a giant claw reaches into the fissure to pull him out, causing rocks to fall all around. Jamie lashes out at the Macra but can do no damage. The creatures are relentless.

Officia tries to turn the gas back on again, but the Doctor thwarts him by simply turning the control back when he moves away. Officia is not fast enough to outpace the Doctor. He'll need help. Oddly, the Doctor encourages him to fetch a guard. As soon as he is outside, Polly closes and locks the door to the gas centre. The Doctor has lifted a second set of keys from Officia and he is locked out. They turn their attention back to Jamie, hoping they have given him the time that he needs to escape.

In truth, he's not free yet, but as the oxygen fills the old shaft. Jamie gains an edge over the Macra. Without their life-giving gas, the creatures weaken and slow down. He continues his struggle, feeling the claw's grip loosen.

Topside, Control has noted the change in gas flow and the voice demands an explanation. Officia calls for guards to help him break into the gas centre. They arrive almost immediately and begin pounding on the door to break it in. The Doctor finally realises that they're not safe and looks frantically for somewhere to hide.

At the far end of the gas centre, near the mass of pipes, three is a heavy set of doors. By closing them, a sort of anteroom can be sealed off around the pipes. Since the only other avenue of escape is the pit, the Doctor and Polly choose the door. They heave on it and it moves very slowly, even as the guards start to break through.

The doors are finally closed and locked just as the guards break in and reach them. At first glance, the area seems little more than a cupboard with a lot of pipes, but investigation shows that the pipes lead off into the darkness. They are the vital gas pipes and they must lead somewhere. The Doctor steels himself and sets out to follow them, Polly close behind.

Officia has turned his attention back to the gas valves and succeeded in restoring them to their original positions. The Macra's vital gas is once again flowing to the old shaft.

But it is too little too late. Jamie has broken free of the Macra's claw and is now haring back up the shaft far ahead of the lethargic creatures. Along the wall he discovers a grating covering an old air shaft or an abandoned tunnel. He heaves at the old grating with all its strength and it comes away in his hands. He squeezes through and disappears.

The tunnel ends back in the colony, a small room where a group of cheerleaders rehearse their inspirational chant. Their leader is lively and positive, urging them on to even more energy and spirit. He tells them it is their job to inspire the brave workers. He makes them start again with even more vigour.

Into the midst of this comes Jamie, trying hard to look inconspicuous and to slip out unnoticed. But the leader catches him and mistakes him for one of the dancers. Jamie tries to put him off but realises he cannot. He admits that he is a dancer. Unfortunately, the leader is eager to see the lad's dance and won't take no for an answer.

Jamie trots out the only dance he knows - the Highland Fling - and begins jumping about with no music. The cheerleaders all clap in time while watching. The leader is pleased but says he's never heard of the Highland Fling. Jamie explains by flinging himself out the door... and straight into Ola and the guards.

Ben is there as well and identifies Jamie as the escapee from the mines, obeying Ola's commands. However, as he sees his friend being tied up and marched away, his mind struggles again to reassert control. The voices are weakening.

Meanwhile the Doctor's path has led him into the old shaft and he and Polly follow the mass of pipes. They are unsure at first where they are going, but soon a faint voice is wafting down the passage. It is the voice of Control, once again warning the guards away from the old shaft as it is in use by Control. In fact, the old shaft is the location of Control.

Before them is a ladder and at its top a window. Climbing up, the Doctor can hear clearly the voice of Control. He can also see the Control area swarming with Macra. The voice of Control urges the human colonists to continue mining the vital gas despite the disruption caused by the strangers. But it is clear to the Doctor and Polly now that the gas is not vital to the colony at all - only to the Macra. It is vital to them as they would die without it.

The Doctor likens the creatures to parasites in the human body, living off a healthy colony as they infect it. Control is the brain of the colony, overrun by Macra. The Doctor concentrates on their gas supply system as a possible weakness, but another idea occurs to Polly - the Pilot. He must see this. Perhaps then he can lead the colony against their invaders.

Jamie has been taken to the Pilot's office to answer for his crime, Ola ready to throw the book at him. However, the Pilot is calm and pleasant as always. He actually seems willing to overlook Jamie's transgression in light of the fact that he appears to have turned himself in. Ola is stunned at this attitude, blaming it for the current laxity of discipline and order in the colony. This laxity allowed the prisoners to escape in the first place. He reminds the Pilot that two of the strangers are still at large.

As if on cue, the Doctor and Polly enter the office, happy and smiling. The Doctor is more chirpy than ever, wishing all a good morning and chastising Ola and the Pilot for their bad-tempered bickering. He appears to have fully embraced the light-hearted tenets of the colony and as such declares that he cannot be arrested. Ola isn't quite sure what to do, knowing that the word of the law agrees with the Doctor. For his part, the Doctor encourages the Pilot to come with him but does not say why.

The eyes of Control are on them and the voice issues from the speakers, forbidding the Pilot from listening to the Doctor or going with him. The Doctor steps up his insistence, aided by Polly. When the Pilot seems on the verge of disobeying Control, the voice becomes nearly frantic, shouting orders to have the strangers arrested.

Ben suddenly hears the voice for what it is - not the voice of Control, but the paranoid voice of someone or something out of control. He struggles with his mind to break through the conditioning.

The Pilot, too, seems to be opening his eyes. He agrees to go with the Doctor. The guards hesitate to arrest the Pilot and allow him to leave with the Doctor. Ola is red with rage but he is so used to obeying orders than he cannot make a move without consulting Control. Instead of pursuing them, he reports the incident to Control via intercom.

Polly and Jamie take this all in. Polly is worried as she sees anarchy breaking out in this well-ordered society. Anything could happen now and they must both be ready for it. They must trust that the Doctor knows what he is doing. At least they are glad to see Ben fighting the mind control. However, they turn to find him and help him but discover that he has slipped away.

Ben has followed the Doctor and the Pilot to the gas centre and hides in the shadows as they pass, heading for the pipes. Ben must resist the voices in his head and the struggle is intense. He does not stop the Pilot as he follows the Doctor.

The Doctor leads the Pilot back toward Control into forbidden territory. He knows he is breaking the law but continues to follow.

In the Pilot's office, Officia has joined Ola. Both men still adhere to the law of Control and believe that the Pilot has been somehow "changed" by the strangers. Control agrees. Its decision: Ola is now in command.

The Pilot has followed the Doctor to the ladder near the Control area. He does not know why he trusts this odd stranger, but he does. They can hear the voice of Control stripping the Pilot of his authority and exhorting the colony to be happy and peaceful - and obedient - once again.

The Pilot climbs the ladder and looks through the window. One look at the creature who have been using the Controller as a puppet and forcing the humans to work for them and the Macra's spell is broken. The Pilot is repulsed by the hideous creatures and the memory of all that he has done under their command floods back to him. His response is immediate and visceral: they must all be destroyed.

Suddenly the voice of Control issues throughout the colony, sounding hysterical. The Macra have spotted the Pilot and the Doctor and order that they be captured and destroyed. As the Doctor and the Pilot race away from the Control area, the Doctor urges the Pilot to defy Control and take full command of the colony once again. It is the only way the Macra can be successfully fought.

Returning to the gas centre, the Pilot and the Doctor are arrested by Ola. He is relishing his new command status. The Pilot tries to explain to him how the Macra are really Control and how they have manipulated the colony for their own ends. Ola reacts as of old - there are no such things as Macra and anyone who says differently is to be arrested. Control joins in, urging Ola to silence him... and the strangers who started all of this.

Control orders that the Pilot and the three strangers be locked in the pipe room at the far end of the gas centre. After a struggle with the guards, this is accomplished. Control orders the gas centre cleared and all citizens to the hall for a music programme. It hopes to placate the people and force them all back into happy obedience. It will be safe to return to the gas centre in precisely four minutes...

Ola obeys and the room is cleared.

In the pipe room, the Doctor, Polly, and Jamie are worried about the precision of this figure. What can be done to them in just four minutes that will make it safe for Control once again? The answer comes quickly - gas! The toxic gas begins to fill the room from a hidden inlet along the pipes and they are all affected immediately. Control begins to taunt them as they cough and try to cover their mouths; nothing they can do will stop the gas. It will kill them within 4 minutes.

Suddenly a ray of hope issues from outside - a knocking on the pipe room door. It is Ben, finally free of the voices and alone in the gas centre. His friends are pleased at his recovery.

Ben tries the door but it is locked and there are no keys about. The Doctor quickly abandons opening the door, knowing that they must defeat the Macra and they have very little time. He sends Ben to the gas flow control desk to identify the inflow and outflow switches. When he does so, he also finds a vital switch just in front of them.

The voice of Control issues frantically from the intercom system. Control can see clearly what he is doing and knows what the Doctor is planning to do. The voice orders Ben to stop immediately, but the voice no longer has any affect on him. He laughs it off and continues to receive instructions from the Doctor through the door.

Ben turns on both the inflow and outflow to full pressure. One flick of the third switch will cause an enormous explosion which will destroy Control, the mines, and all the Macra in one go. Control continues to try and stop Ben, implying that the humans will be destroyed as well, but he's not buying it. Control calls out the guards to stop Ben, but they are far too late.

At the Doctor's order, Ben throws the switch and the explosion is swift and sudden. All the equipment explodes and the rumbling and destruction can be heard deep into the mines. The Macra are all destroyed.

Some time later, life has returned to normalcy in the colony. A festival is going on in celebration of the liberation of the colony. The Pilot thanks the strangers for their help in saving the colony and names the Strangers' Trophy to be awarded to the winner of an annual dance contest. Things are really back to normal here. How they will continue to be happy and pleasant without the enforcement of mind control is unknown.

Ben is agitated, having learned something that he knows the Doctor will want to hear. He interrupts the Doctor as he sits and watches the festival, enjoying the unusual leisure time. However, Ben reports that he has heard news that they will all be asked to join the colony and that the Doctor himself will be made the next Pilot! This is alarming news indeed and the Doctor immediately falls out of the spirit of the proceedings, wishing to get out of here as quickly as possible. The only way they can is through the revelers.