The Doctor lies unconscious on the floor of the TARDIS. Ian is flung over a chair and Susan is slumped over the console. The ship is quiet and motionless. The main lights are out and the console room is illuminated by isolated pools of light. Barbara enters from the living quarters, staggering and blank-eyed. She has a blanket draped around her like a giant scarf. She wanders past the Doctor and Susan, finally stopping before Ian's chair. She looks at him as if trying to remember who he is. Indeed she speaks his name - finally connecting it with his face - as if she barely knows him. Her strained thinking is interrupted when Susan awakes and slides off the console and onto her feet. She, too, is blank-eyed, but whatever is wrong with her is much deeper. She has trouble walking and focusing. She slides forward and gazes hard at Barbara, trying to recall where she knows this woman from. Suddenly, Susan's head convulses and she grabs at the back of her neck, complaining of a sharp pain there. Just as suddenly it is gone. Susan is back to her stunned state.

Susan catches sight of her grandfather lying on the floor with a gash in his head. This seems to snap her out of her zombie state and she goes to him. Barbara kneels to help and Susan is up immediately to fetch some ointment. The pain in her neck returns and she must stand still. She catches sight of Ian but does not recognize him. Neither of the women can seem to muster much concern for the men, but Barbara does hurry Susan along for the ointment and some water. Susan staggers back to the living quarters.

Barbara turns to find Ian awake and standing. His face is blank and he addresses "Miss Wright" as if he just found her working late in her classroom at Coal Hill School. He seems to come back to himself for a moment, a look of panic on his face. Then he's back to confusion, inquiring about the stranger lying on the floor.

In the living quarters, Susan locates a first aid kit. From it she produces a strip of white bandage with intermittent colour strips all along it. She uses a pair of long metal scissors to cut off a length of it.

Ian and Barbara "examine" the Doctor's injury. Ian says the gash doesn't look that bad, but his tone - and Barbara's for that matter - indicates he is not thinking clearly or taking this seriously at all. He speaks in a monotone and she like a child. Ian is on the edge of giddiness. The Doctor stirs and mutters that he can't take Susan back, as if in a dream. This amuses Ian but it seems to spark something in Barbara's addled mind. She suddenly recalls the TARDIS. Ian is still very confused.

Susan activates the food machine to get some water. A light indicates that it is empty, but it still produces a plastic pouch of water. As dazed as she is, she knows this is not right. But she takes the water and the bandage and returns to the console room. She gasps as she sees the main doors standing open of their own accord. Again this crisis seems to clear her mind for a moment as she draws Ian and Barbara's wandering attention to it. Ian thinks the Doctor may have opened them earlier and Barbara thinks the ship must have crashed, but Susan knows both these things are impossible. She realizes for a moment that they aren't thinking clearly and tries to revive her grandfather, but then her face changes again as her own mind succumbs to whatever influence pervades the ship. But this time she is not a rambling zombie; this time paranoia takes hold and she announces that something has gotten inside the ship.

Ian and Barbara dismiss this speculation. Barbara takes the strange bandage strip and starts to place it around the Doctor's head. Susan explains that the coloured parts are medicine which will disappear as they go into the wound. When the bandage is all white, the wound has healed. Meanwhile, Ian has approached the doors in a daze. When he nears them, they close tight of their own accord. When he moves away, they open, then close again as he approaches. This time, they remain closed.

Susan is seized by some sort of mania and rashly decides to try the door controls. She touches the panel and is flung away from the console with a scream, landing unconscious on the floor. She has received an electric shock but this does not immediately register with Ian and Barbara. For a brief second, realization of all that has happened seems to break through Ian's clouded mind. He shakes his head to try and clear it. All that they've said, must of it ridiculous, plays back in his head and he shouts out loud trying to make sense of it all. But his lucidity is gone just as quickly as it came. When the Doctor begins to stir, Barbara instinctively goes to care for him and instructs Ian to carry Susan to her bed to rest. Ian warns her to be careful here on her own, but in their current state of confusion, neither of them knows what it is they must be careful of.

The Doctor complains of a sharp pain on the back of his neck, the same as Susan had earlier. Barbara can find no sign of injury there.

Ian places the unconscious Susan on her bunk and then goes to find water to wet his handkerchief. Despite his spotty amnesia and cloudy thoughts, his instincts still seem to be intact. He goes to the food machine and dials up water. Again it registers empty yet produces water just the same. Returning with the handkerchief, Ian finds Susan awake and kneeling on her bunk, holding the scissors like a dagger. Her face is tight and there is a wild look in her eyes. She is in the grip of full-blown paranoia. She does not recognize Ian (or is it that she thinks this isn't really Ian at all?) and attempts to stab him when he tries to take the scissors away. But when she moves to strike, as if possessed, there seems to be some internal struggle going on to control her hand. She stops moving forward and stabs harmlessly at the bunk instead. She passes out and the scissors fall from her hand.

Some time later, the Doctor, Ian and Barbara sit in an alcove discussing the situation. The scissors sit on a table nearby, well out of Susan's reach. The Doctor dismisses Barbara's suggestion that they've crashed, believing instead that they've landed somewhere. Barbara, beginning the first signs of paranoia, lashes out angrily at the Doctor's imprecise guesswork and goes off into the gloomy console room. Ian inquires about where they've landed, but the Doctor is more concerned with why. The base of his neck still pains him as he rises to check the controls. But he too is seized by the beginnings of paranoia and suddenly demands to know if either of them have touched the controls. This subsides quickly and he regains himself, staggering as his clear mind recalls all that has been said and done. He listens carefully as Barbara suggests the possibility of some alien intelligence having invaded the ship when the doors were open, amplifying Susan's earlier suggestion. She seems almost clear-minded, but Ian is still cloudy and the idea amuses him. With this, Barbara's mind slips back and the Doctor's goes forward again into paranoia. He dismisses the idea out of hand and chastises Barbara for wasting his valuable time. He heads off to check the ship's systems.

Ian's mind seems to be clearing and he agrees to help the Doctor. But he warns the Doctor to stay away from the console as it gave Susan an electric shock earlier. This is the first memory he's been able to clearly retrieve since this whole business began. Ian warns Barbara not to tell Susan about her alien intelligence theory, unaware that the girl has slipped out of bed and is listening to them at this very minute. Unseen by the others, Susan grabs the scissors from the table and slips back to her bunk. She is consumed more than ever by her feelings of paranoia.

Ian joins the Doctor at the fault locator, his mind starting to slip back into its earlier hazy state. He struggles to hang on as the Doctor instructs him to read off the fault indicators if and when they register. He finds it hard to concentrate on the letters and numbers.

Meanwhile, Barbara goes back to check on Susan. The girl is awake and staring suspiciously at Barbara. At first she will not speak. Barbara suspects a recurrence of amnesia initially, but when Susan peppers her with accusatory questions, she realizes instinctively that something else is wrong. Barbara has also noticed the scissors missing from the table and tells Susan to hand them over to her. Her deception uncovered, Susan pulls the scissors from hiding and brandishes them menacingly. She reveals what she overheard and accuses Barbara of lying to her, especially as it was Susan's idea in the first place. Barbara's duplicity is clear to her. She and her grandfather are in danger, from their companions! But when Susan moves to attack Barbara with the scissors, she is again seized by some internal force which cannot let her harm Barbara. Her grasp weakens and Barbara is able to pry the scissors from her hand.

The energy seems to drain from Susan's body and she collapses back onto her bunk. Her mind clears momentarily and she notices the shadows and the silence. She is very frightened. Barbara is still clouded and returns to her alien intelligence theory. This talk spurs Susan back into paranoia mode and she provides further grist for the theory - the intelligence could have got in while the doors were open and could now be hiding inside one of them. In a brief moment of lucidity, Barbara tries to get her to stop talking about the absurd - yet frightening - idea.

Ian joins them, announcing that their check of the fault locator indicated nothing wrong anywhere in the ship. But Susan is barely listening to him. She is clearly frightened by him, her fear and paranoia growing by the second. She wonders where her grandfather is, her tone suggesting that Ian may have harmed him. Ian announces that the Doctor has decided to operate the scanner to see if the "fault" lies outside the ship. At this, Susan screams and bolts from the living quarters.

She comes to warn the Doctor not to touch the console. This excitement clears her mind for a moment and she recalls the electric shock and the pain at the base of her neck. The Doctor, too, recalls this pain, but the newly arrived Ian and Barbara did not experience this. For a brief moment, the two groups are polarized in opposition. A sign of things to come.

Tentatively, the Doctor touches the scanner switch. He does not receive a shock and the scanner activates immediately. On it is a picture of a grassy meadow with trees dotted around. Barbara and Susan think it looks like England, but the Doctor - thinking clearly for the moment - sees that this is only a photograph and not a true image of what is outside. Without warning, the doors glide open. A brilliant light and a roaring noise issue from outside but they can see nothing. The Doctor orders Susan to close the door but Ian reaches the switch first. The doors close and the Doctor glares accusingly at Ian. His proximity to the switch singles him out as the person most likely to have opened them in the first place.

The group is distracted from this when another picture appears on the scanner. It is a lush but alien-looking jungle. Susan and the Doctor recognize it as the planet Quinnis in the Fourth Universe, where they nearly lost the TARDIS on a visit. It, too, is just a photograph however. The Doctor wonders if this could have anything to do with the TARDIS memory bank, which records all their journeys. Ian, beginning to slip into paranoia like the others before him, wonders why the Doctor has never told mentioned this memory bank before.

Shortly, a third picture appears - a planet suspended in space. Then the scene changes to a farther look at the same planet, then a distant shot of a constellation, followed by a blinding flash meant to indicate an explosion. It makes no sense to anyone, but when Ian asks the Doctor for a guess, the old man turns on him mockingly and asks Ian himself to tell them what it all means. There is full-blown paranoia on both sides. Barbara, thinking a bit more clearly, tries to suggest opening the doors to take a look outside, but the Doctor viciously changes tack on her, reversing himself and announcing that what is inside the ship is most important now. But all that is in the ship are the four travellers.

The Doctor now accuses Ian and Barbara of being the cause of this entire disaster. He is almost ranting as he accuses them of knocking him and Susan out, then tampering with the controls. Their motive: blackmailing him into returning them to Earth. It doesn't take long before Barbara is fed up with his ridiculous theory. She lights into the Doctor, reminding him how Ian saved his life by making fire in the Cave of Skulls and how both she and Ian risked their lives for him and Susan while fighting the Daleks. All because of his trickery which got them to the city in the first place. Instead of gratitude or common sense, the Doctor can only offer insane accusations. She storms off in anger but stops with a scream in front of the decorative clock on a pedestal near the console. She screams in terror, near hysteria.

The entire clock face - hands, numbers, everything - has completely melted away, revealing the workings within. It is a grotesque sight. All four of them look at the clock in stunned fascination. Ian checks his watch and it is the same. Barbara's as well. She cannot rip the horrifying object from her wrist fast enough. She tosses it across the room and collapses in a chair, sobbing in fear and shock. This puts an end to the earlier argument but a charged silence reigns. No one has forgotten what was said.

Ian notices the Doctor is missing, but he joins them a moment later bearing four cups on a tray. He calls it a "nightcap" to help them rest. They will think more clearly for some sleep. They are all overwrought. Ian, still in the grip of mild paranoia, questions the Doctor's abrupt change of manner but gets no answer. Barbara bolts from the room, somehow instinctively knowing that sleep is the best thing for them now. Susan, thinking clearly for a moment, urges her grandfather to make amends with Barbara then heads off to sleep herself.

Ian, too, urges him to apologize, feeling that the danger they face will be made worse by this personal quarrel. But his voice is too hard and the Doctor recognizes the tone of accusation. He does not accept the advice. He says there is no time now for manners. He needs time to think - a clear message from his subconscious to combat the cloudiness and paranoia overtaking him. He refuses to waste time on pleasantries. Ian follows him back to the living quarters and the Doctor - slipping back into paranoia - accuses Ian of trying to get "one jump ahead" of him and take over the group. He assures the young man that this will never happen, not while the Doctor possesses all the wisdom and experience of his time and his travels. The Doctor speaks it as a warning as he goes off to sleep. Ian takes it as something even more sinister. He is unaware that Susan has been listening to this heavily-charged exchange.

Susan slips back into the bunk area she shares with Barbara. Barbara is still awake and staring at the ceiling. Susan apologizes for the Doctor's accusations, but instinctively supporting his course of action no matter how irrational it appears. But Susan's words are a small pebble of rationality in an ocean of acrimony and distrust generated by...something which has firm control of the travellers' minds. Barbara refuses to listen and closes her eyes to sleep.

Some time later, all is quiet in the TARDIS. Ian, Susan, and Barbara have drunk their "nightcaps" and are asleep. The Doctor has drugged them all to give him time alone to find the cause of this disaster. The paranoia clouding his mind cannot override his innate intelligence and urge to solve the problem. But it is impairing his judgement. He moves though the sleeping quarters making certain his trick has worked and his companions are asleep. Satisfied that they are, he moves quickly to the console, trying to figure out what to do next.

The Doctor's eyes lock with Ian's. The young man's eyes are not wild, but vacant. He is being compelled by the strange forces clouding all their minds, with the addition of the Doctor's sleep drug. The Doctor's intense gaze breaks through Ian's mental haze and he realizes what he's doing. He lets go immediately and staggers backward with a strangled cry. Ian collapses unconscious on the floor. Barbara, awoken by all the commotion, runs to Ian's side, her mind clearing quickly.

The Doctor takes this action as confirmation of his earlier theory. In the grip of full-blown paranoia, he blames the entire crisis on Ian and Barbara acting out of pure malice. Barbara's clarity is nearly complete and she pleads with the Doctor to realize that something is very wrong. Their clouded minds, their paranoia, their irrational words, and even their physical actions are being influenced by some force. She tries to make the Doctor see, but he is blinded by paranoia. He cannot even see his own irrationality. Shortly, Susan joins in, siding at first with her grandfather against the others. She, too, is blinded.

But all this changes when the Doctor announces he will henceforth treat Ian and Barbara as enemies. Realizing what this could mean, Susan seems to clear a bit. She goes to Barbara, who is frantically trying to rouse Ian, and tries to understand what happened. Barbara seizes on this, her mental clarity still holding, and she tries to make Susan understand that they couldn't and wouldn't try to harm her or the Doctor. Susan is starting to waver and the Doctor accuses Barbara of trying to poison Susan's mind against him. But when Ian awakes for a moment to shout a slurred warning to the Doctor not to go near the controls, Susan does believe them. It is clear to her that Ian wasn't trying to hurt the Doctor but to keep him away from the electrified controls. In his clouded state of mind, exacerbated by the sleep drug, it came across as a crazed attack. She tries to make this clear to her grandfather.

But the Doctor is convinced of their treachery and will not listen. He has only one recourse to save himself and his granddaughter - to put the others off the ship. Susan begs him to reconsider, warning that the outside atmosphere may not support human life. The Doctor does not care - he is doing what he must. His paranoia is still in full force as he badgers the semi-conscious Ian to leave the ship immediately. Still dazed, Ian tries to stand, thinking he is going for a stroll. Susan tries again to stop her grandfather, noting that Ian is too ill to even stand.

Suddenly, a foghorn-like alarm goes off. The Doctor and Susan react with horror. Susan tells Barbara it is part of the ship's warning system. She and the Doctor race to the fault locator, where they are horrified to find the entire system lit up, indicating that every part of the ship has a fault at once. Barbara watches, inching toward the console to get a better look. Suddenly Ian grabs her neck from behind, just as he did to the Doctor earlier, muttering a warning as he tries to pull her away from the console. The meaning of his earlier action is now perfectly clear. Ian slumps back down unconscious.

The Doctor approaches Barbara, all the excitement clearing his mind again. Much is clear and he hasn't much time to explain. But he must first assure Barbara that he no longer means her any harm. He explains that the fault locator can indicate by a small light the minutest failure in any of the TARDIS' systems. But now the entirety of the fault locator has lit up, indicating that the entire ship is on the point of disintegration. They are all to blame, and they are all in danger.

As if a spell had been lifted, all 4 are fully cognizant for the first time since the initial explosion. Ian comes to, explaining his attempt to keep the Doctor from being electrocuted. The Doctor cuts him off, admitting that he had jumped to the wrong conclusion. They are a team once again.

Susan reports that the fault locator's indicator lights are flashing in full every 15 seconds. She had to count the time herself as all the clocks are melted. She goes back to keep watch again. The Doctor knows this is bad news - it indicates some force of enormous magnitude is acting on the ship. It is so strong and all-encompassing that it can put every part of the ship out of action at the same time. They are on the brink of destruction. They haven't crashed - that much is clear - nor is there an intelligence inside the ship. And it is certainly not Ian and Barbara's sabotage causing this. Whatever is threatening them is outside. And they must find out what it is.

Panic begins to overtake Ian as he realizes the extent of the danger. But Barbara remains calm and very clear-headed. When Susan returns, noting that the fault indications are still 15 seconds apart, Barbara realizes what it means: that they still have "a measure of time" for as long as it lasts. Time was "taken away from them" when the clocks all melted, now it is being "given back" through the indications of the fault locator. Because it is running out!

Another explosion rocks the ship, accompanied by a flash. All of the travellers reel. There is even a sound like the beginning of the ship's dematerialisation. All eyes go to the central column of the console, which is settling back after having risen alarmingly high. But the main power in the ship has been off since the initial explosion and the column should not be able to move at all. The Doctor and Susan are horrified at the implication of this. The massive power of the TARDIS, enough to propel them through all of time and space, is stored beneath the column. It now appears to be trying to force its way out by pushing the column up and out. They and the ship will be completely destroyed.

The Doctor announces with certainty that they have only 10 minutes left until "the end". A stunned silence falls over the group, unable to believe the inevitability of their demise.

The Doctor, however, still wrestles with the problem, unwilling to give up. Soon the others join him. He goes to the console, being sure to stay near the panel that is not electrified - the one containing the scanner switch. But why is that one panel safe? The pressure is too much for Susan and Barbara must go to her and calm her down. The Doctor only wishes he had some clue as to what is happening.

But Barbara has deduced that they have been given clues all along. The melting clocks first made them aware of time by taking it away. Now it has been returned by the cycling of the fault locator. She believes it is the ship - the TARDIS itself - using its defence mechanism to warn them of the danger. There is no real fault with the ship - the fault lies with them. The TARDIS is not alive, but the Doctor agrees that it can think as a machine would think. Barbara's theory has gained in plausibility. She believes that the nature of the danger lies beneath the control column and with the power stored there. It is trying to force its way out, but why or how? Ian suggests some magnetic force, but the Doctor says it would have to be as powerful as an entire solar system to affect the ship's power source. As if in validation of the Doctor's theory, another flash and explosion rock the ship. To Barbara it is clear. The ship is warning them the only way it can.

Ian wonders if the power escape is too far advanced to stop, but the Doctor assures him they would all be dead instantly if that was the case. This is no ordinary disaster. Everyone rallies around Barbara's theory, Susan suggesting that the console's electrification is a clue as well. The Doctor agrees it's possible, but calls it a long shot. Another explosion rocks the ship, but the Doctor responds matter-of-factly. He sends Susan and Barbara to the doors in case they open, so they can look out and see what is really there. To Ian, however, the Doctor confides that they have far less time than he initially thought - 5 minutes only - and when the end comes he doesn't want the women to know anything about it. Ian agrees to face the danger side by side with the Doctor, both despairing of a solution and both lying to the women about their conversation.

With no other course of action available, the Doctor activates the scanner. The succession of pictures is the same as before - England, Quinnis, planet, solar system, constellation, explosion. During the England picture, the doors open of their own accord. There is nothing outside but space despite the pictures. When the picture of Quinnis appears, the doors close. Barbara - thinking quickly - connects the two. The doors open when a "good" picture shows, as if it were safe to go out, then closing when a "bad" picture shows, warning them to stay inside. As the rest of sequence plays out, ending in destruction, the Doctor knows what it means. The last part is their current journey, into the heart of a violent explosion. The ship's defence mechanism refused to allow the ship to be destroyed and so stopped it in space. The ship itself has been trying to warn them ever since in the only way it could. Another explosion rocks the ship, throwing the Doctor against the safe part of the console.

Like a man possessed, the Doctor explains what is happening - what has become so completely clear to him. The ship has travelled to the beginning of a new solar system where all the matter is rushing past them and coagulating into denser and denser mass. Instead of going back to the very beginning of the universe, the explosion there capable of destroying the ship, the TARDIS itself stopped short. However, the increasing gravity new solar system is now trying to drag the ship's power source free, a situation which will also destroy them. The powers of creation and destruction together fascinates the Doctor.

Ian is still bent on finding a solution to the problem in the moments they have left. He begs the Doctor to recall what he did to the controls to put them here. Finally, the Doctor remembers the fast return switch he activated to bring them back from Skaro in the future. The Doctor takes precious seconds to find a small penlight and locates the switch on the console - on the safe panel near the scanner controls. A quick glance shows him that the switch is still activated, pressed down all the way. Pulling off the cover, he finds the spring beneath the button hasn't released itself from its previous use - it's still on full. He releases the spring and suddenly the ship comes back to life - the console lights wink on, main lighting is restored, and the familiar mechanical hum begins again, rising in pitch until it is normal.

There is a moment of stunned disbelief before the travellers can accept that they have survived. Ian congratulates the Doctor with a clap on the shoulder and Susan hugs him in relief. The Doctor explains to Susan about the stuck spring and the fact that since the switch wasn't actually broken, the fault locator couldn't indicate what was wrong. It was simply as if the fast return switch was being pressed down continually. The Doctor speculates that he might be going round the bend just a little!

Susan is glad the danger is over, but notices Ian and Barbara standing off by themselves. She knows that it is not over for them. She reminds her grandfather of the terrible things they said and did to Ian and Barbara. The Doctor stumbles over his conciliatory words but Ian stops him. He knows that what was said was under the influence of the enormous energy forces outside and the head of the moment. Ian's forgiveness is as tacit as the Doctor's conciliation. But this will not do for Barbara. The Doctor knows he must make it up with her but cannot bring himself to apologize for what he said under the influence of the power. Or is it that he cannot really apologize at all? The Doctor clumsily congratulates Barbara on solving the mystery - reading a pattern into the ship's clues and sticking with them. She saved all their lives with her intuition and intelligence. Barbara is overwhelmed by their recent brush with death. She cannot speak and goes off into the living quarters alone still looking quite distressed.

The Doctor admits that he has underestimated Barbara in the past, but no longer. Then he's back at the controls, ready to try again to get Ian and Barbara back home. But he hesitates a bit before activating the controls and Ian laughs at his "once bitten" reaction. Shortly the ship dematerialises and is on its way again.

Some time later, having shed his now-white medicinal bandage, the Doctor approaches Barbara as she sits on a couch in the living quarters. She has cleaned up and changed clothes, but sits silently now staring off into space. He tells her they've landed on a planet that seems quite safe, but the temperature is rather cold. He knows that Susan's told her this already, but he's using it to try and open up their communication again. It is clear that she's still very angry at the things he said to her. Without apologizing, the Doctor does express regret at threatening to put her and Ian off the ship. As they learn about each other, he says, so they learn about themselves. Because Barbara was determined to prove that she and Ian weren't responsible for sabotaging the ship, she set her mind to the problem at hand and solved it. They are a team, all with separate strengths which will see them safely through these little adventures.

Susan joins them, dressed in boots and a heavy coat. She is ready to go outside and asks if they are all coming. The Doctor looks to Barbara, who finally says that she is ready to venture out again. All is well. The Doctor helps Barbara on with a heavy coat from the wardrobe, saying that they must look after her as she's a very valuable member of their team. It is the closest he will ever come to an apology and Barbara knows it. With a smile, he offers his arm to escort her out.

They enter the console room where Ian is putting on a grand overlarge Ulster before going out. Barbara is complementing it when Susan rushes in from the open main doors and pelts her with a snowball. Barbara runs after her, set on revenge. Outside the doors is a mountainous terrain covered in snow. But the day is clear and sunny. Ian learns from the Doctor that his Ulster came from Gilbert & Sullivan. Proudly, Ian starts for the door.