A desperate and terrified old man stumbles to a communications console and sends out a cry for help. But his tormentor, a maniacal figure like an insane court jester, finds the old man and cuts off the transmission before he can give any details about the terrible threat...

In the TARDIS, Peri is trying to teach Erimem to read. It’s a frustratingly slow process for them both, and the irritated Erimem eventually begins to question the plot holes in her book, Alice in Wonderland. The TARDIS then shudders about them, and Peri and Erimem head for the console room to find out what’s happening. The Doctor is trying to find out as well; the TARDIS has put down somewhere in interdimensional space and will not dematerialise. The Doctor tries fiddling beneath the console with his Molenski Univarius, an all-purpose sort of Time Lord Swiss Army knife, which he claims can fix anything -- prompting an acerbic comment from Peri about the chameleon circuit, much to Erimem’s confusion.

Despite the Doctor’s objections, Peri operates the scanner to see what’s outside, and sees a strange landscape of bizarre angles and twisted perspectives. Peri and Erimem press the Doctor for an explanation, and he reluctantly tells them that they’re in the Axis, the hub of a spindle of dead-end timelines in which history has spiralled off in catastrophic directions and must be cut off from the rest of reality. This is usually the result of meddling by the Time Lords, and the immortal, omnipotent Overseer of the Axis thus has little love for the Doctor’s kind. The Doctor is, however, forced to concede that he may have been brought to the Axis because there’s something wrong with it, in which case the Overseer may need his help. In case of trouble, the Doctor programmes a default co-ordinate jump into the console, and leaves, telling Peri to use the emergency jump only in the direst of emergencies -- and under no circumstances whatsoever to leave the TARDIS or let anyone but the Doctor inside. As he leaves, Erimem questions Peri about her earlier comments, which implied that there are other TARDISes in existence.

Outside, the Doctor is surprised by the sudden appearance of the jester, who claims that he’s here to lead the Doctor to the Overseer. The Doctor follows the jester through increasingly bizarre, Escheresque chambers, and gradually becomes more and more disturbed by the jester’s mad behaviour -- particularly when the jester claims to be “the new broom”. In one chamber, the Doctor and the jester appear to be walking upside-down while invisible and intangible dancers caper about them; in another, a group of children are reading closed books with their eyes shut while a young girl makes impossibly complex shapes out of building blocks. The jester calls this the process of creation untrammelled, but the Doctor sees only chaos and insanity.

Time seems to be dragging by in the TARDIS as Peri and Erimem wait for the Doctor to return, but that may just be their perceptions. It’s hard to tell whether any of what they can see on the scanner is real, and Erimem eventually convinces Peri to go outside with her, just for a moment, to see for themselves. Outside, they find their surroundings shifting about them, passageways moving or disappearing, and though they’ve only taken a few steps, the TARDIS is on the other side of the room already. Peri insists upon returning to the TARDIS before they get lost, but before they can do so a screaming man rushes by, his clothing aflame. Peri is reluctant to get involved, remembering her promise to the Doctor, but Erimem refuses to stand by while a man is in need of help -- and as she approaches the injured stranger, a fire-breathing dragon thunders in. Erimem can’t carry the man back to the TARDIS by herself, and Peri has little choice but to help.

Inside the TARDIS, Peri shows Erimem how to shut the doors. The dragon continues to attack, and the man they have saved, Tog, warns them that by saving him they have doomed themselves; the Firebreed will not give up until it has killed its prey. Tog is from a world called Pangorum, which was once a peaceful and beautiful land until the Firebreed, monsters from myth and legend, suddenly reappeared and began attacking the population. Suddenly, the world itself seemed to turn against its people, and the beauty of Pangorum was forever lost to unspeakable horror. Tog was one of an expedition who followed rumours of dark magicks to a research facility in the northern wastes; there, they found a patch of empty space which led them to the Axis, but when they passed through, they were hunted and picked off one by one by a mad creature like a jester. The jester, who has the power to twist reality at its whim, called itself the Overseer.

Peri and Erimem are shocked by this news. Either the Overseer has gone mad or the jester is not the real Overseer; either way, the Doctor is in danger. Tog insists upon returning to Pangorum to find others willing to fight this enemy; he has accepted that his world may be doomed, but he will not permit this madness to spread further. Peri is unwilling to abandon the doctor in the Axis, but it seems she has no choice; two more Firebreed have joined the first outside, and the TARDIS is beginning to heat up from their attacks. There’s no telling how much more it will take, and as this seems to qualify as the direst of emergencies, Peri reluctantly operates the emergency jump procedure. The TARDIS dematerialises -- without the Doctor...

The jester kicks the Overseer out of his throne and takes his place, and the horrified Doctor realises, much too late, that the jester has escaped from one of the festering timelines around the Axis and has infected the Axis itself with his timeline’s insanity. He also realises that the jester allowed the Overseer to send the distress call which lured the Doctor here -- because in order to escape from the Axis, the jester needs the Doctor’s TARDIS.

The TARDIS materialises once again -- and to Peri’s relief, they’re still in the Axis. The TARDIS is still unable to leave the Axis, and the failsafe simply moved it a short distance out of harm’s way. Thus, Peri still has a chance to find the Doctor and warn him that the Overseer is not what he seems. Tog agrees to help her find the Doctor on condition that they then return Tog to his homeworld to stop the spread of its madness. Peri shows Erimem the safety combination on the console, and she and Tog emerge from the TARDIS and set off in search of the Doctor. Somewhere in the madness of the Axis they find a marketplace, where a leering trader tries to sell Peri a pair of shrieking birds; one represents the aspirations of all living beings, and the other carries the woes of the world on its wings. Tog warns Peri that the jester is trying to distract them with nonsense, but it’s working; the Firebreed have their scent once again...

The jester calls up an image of the dead-end timelines in an oracular pool, and admits that he’s meddled with them, creating and destroying worlds on a whim. However, since these timelines are cut off from the primary flow of history, all of his changes eventually reset themselves. He’s in the mood for something new and unfamiliar, and now it seems he may get it, for Peri has disobeyed the Doctor’s orders and left the TARDIS. Despite his control of the Axis, even the jester doesn’t know what will happen next. As the Doctor rushes off to the rescue, the jester changes into the form of a woman -- perhaps his true form. Outside the oracle room, the Doctor finds himself on a platform overlooking the rest of the Axis, but the rickety platform begins to collapse beneath his feet, and he has no choice but to descend into the insanity of the Axis to look for Peri directly.

Peri and Tog have left the marketplace for a carnival, but Tog warns Peri that the gay colours and games are just distractions from the evil at its heart. Nevertheless, Peri is enticed by a barker into playing a game of skill by bowling a ball down a chute, and Tog barely gets her clear in time before the ball reaches its target -- and explodes. Tog’s compatriots all fell victim to similar tricks, and now Tog has been numbed by the horrors he’s seen; all he desires is to ensure that the madness that engulfed his world spreads no further. The Firebreed then chase him and Peri into the carnival’s funhouse, where a flash flood roars down the hallways and sweeps them both away.

The Doctor also finds himself in the carnival and hears the Firebreed roaring in the distance. An automated fortune-teller directs him to the funhouse, where he is attacked by a unicorn. However, the jester does not respond to the Doctor’s angry shouts; perhaps, having set this insanity on its course, the jester is now occupied elsewhere. Inside the funhouse, the Doctor is reunited with Peri and Tog, who have just been washed up on dry land. As they search for a way out, Tog tells his story to the Doctor, who becomes interested when Tog describes the “empty space” through which he entered the Axis: a rip in the fabric of space and time. Tog tells of his expedition to the northern wastes, following rumours of forbidden experiments to an abandoned research facility that appeared to be the source of the chaos visited upon their world. The Doctor, however, urges Tog to tell his story on the run, for he’s worked out where the jester is going -- and why.

Erimem is beginning to become wary of the Doctor, who is alternately quiet and manic and speaks of the desperate, dying stench of rotting, dead-end timelines. When the TARDIS materialises in the abandoned research station on Pangorum, the Doctor immediately sets about repairing the equipment, restoring heat and light to the building. Noting his odd familiarity with the controls, Erimem asks the Doctor to take her back home to Chicago when this is all over, and he agrees to do so. He then sets off in the lift to look for spare parts, and Erimem, who now knows that this is not really the Doctor at all, tries to look for a way out.

However, before Erimem can find an escape route, her captor returns -- this time in her original form, that of the madwoman Jarra To. Jarra To changes shape before Erimem’s eyes, transforming into the jester, the market trader, and the carnival barker, all telling the terrified Erimem that she, Jarra To, is about to bring the world to an end -- and that she’s brought Erimem here simply to watch. Erimem realises that Jarra To has been confined to this dead-end, stale timeline for so long that the predictability has driven her insane, and she craves chaos and uncertainty above all else. She also realises that Jarra To is responsible for whatever event cut this world off from the rest of reality. Furious, Jarra To strikes Erimem down, knocking her unconscious.

The real Doctor is stuck in the Axis with Peri and Tog, and without the Overseer or the jester to hold the Axis together, it’s slowly being crushed under the weight of the timelines it supports. Within an hour or two, it will collapse completely, and the insanity of the corrupted timelines will spread out into the real Universe. Only another of the Overseer’s kind can save matters, but the Doctor fears that the jester will have anticipated this. Worse, the Doctor can’t sense the TARDIS anywhere in the Axis -- but how could the jester have known how to pilot such a craft?

As the Doctor, Peri and Tog return to the oracle room, Tog tells the Doctor about the madness that engulfed Pangorum, and of the madwoman responsible -- Jarra To, who unleashed horrors upon their world in her quest to reconcile science and superstition. Screams echo through the Axis as the walls between the dead-end timelines collapse; some of these broken histories have been festering for aeons, and their inhabitants, all quite mad, now see the end coming. The Doctor is now forced to admit to Tog that the world he knows is also an illusion, a twisted history that should never have come about. Tog understands, and accepts that the best he can hope for now is put his world out of its misery before its madness spreads to all others.

The Doctor, Peri and Tog then reach the oracle room, only to find that the jester has smashed the communications unit. It will take too long for the Doctor to repair it, and Tog thus suggests returning to Pangorum via the portal which Jarra To opened. The Doctor concedes that she may have returned there to finish off her work, but when Tog leads him back to the portal, they find two open doors. As Tog tries to choose the right one, the Doctor is attacked by a feral madman from one of the infected timelines. Tog and Peri manage to overpower the madman, and Tog chooses one of the two doors -- but when he and his friends pass through, they find that Jarra To has moved the portal’s exit out of the research facility and down into the magma fields, the nesting ground of the Firebreed.

The Doctor, Peri and Tog are confronted by a mother Firebreed and her young offspring, and their retreat is blocked by the magma. The mother Firebreed herds her prey towards her child; they are to be its first kill. Fortunately, Peri realises that the Firebreed cannot see clearly, and the Doctor uses his Molenski Univarius to generate a subsonic signal which causes the Firebreed much distress. As he and his friends flee, they spot the research facility towering above the magma, and realise that Jarra To must have placed them here so she could watch them die. She might still get her wish; the Univarius is running out of power, and the Firebreed, driven to distraction by its subsonics, are starting to claw through the rock and expose more magma.

The jester mocks Erimem and wonders aloud why the Doctor would choose to travel with such lesser, dim-witted beings; perhaps he thinks of Peri and Erimem as his pets. Erimem tries to fight her way past the jester, but s/he transforms back into Jarra To and drags Erimem back into the lift, laughing out loud when she realises that Erimem got trapped in the basement because she couldn’t read the lift buttons. Since Erimem refuses to play along by asking Jarra To inane questions, Jarra To transforms herself into a copy of Erimem and asks the questions of herself, revealing that she intends to reactivate the machinery that she used to punch a hole out of this reality and into the Axis. This time, however, she intends to use it to destroy this entire timeline, leaving no trace of her origins behind, while she escapes to travel the Universe in the Doctor’s TARDIS.

Erimem, not the fool Jarra To thinks she is, questions why Jarra To would go to such trouble to steal the Doctor’s TARDIS when the dead Time Lord in the basement presumably had one of his own. She soon deduces that the “chameleon circuit” which Peri and the Doctor mentioned earlier must be functioning properly in the dead Time Lord’s TARDIS -- which means that Jarra To has no idea where it is or what it looks like. Furious, Jarra To lashes out at the Time Lord for arrogantly walking into her laboratory and trying to shut down her Timescoop experiments, even if they had gotten “a little out of hand”. When she turned the tables on him and sucked his brain dry, the other Time Lords shut down this timeline completely, leaving her to rot within. But now she has a means of escape. She’s amused by the TARDIS’ police box shape, which seems to imply that the Doctor considers himself a cosmic policeman. Jarra To enters the TARDIS, leaving Erimem to die in the collapse of this timeline. Instead, Erimem returns to the basement, having realised that the dead Time Lord must, like the Doctor, have a key to his TARDIS. She steels herself, searches the fetid corpse, and finds a key in the shape of an ankh.

Outside, the stampeding Firebreed break through the crust of the magma field just as the Univarius runs out of power. The Doctor, Peri and Tog flee for their lives, but as the Doctor nears safety, the heat becomes too much for him and his brain shuts down temporarily. He soon recovers -- and finds that the Firebreed did not attack during his moment of weakness, which suggests that they are being kept at bay from the facility. He and his companions then hear a rumbling sound like an approaching storm, and the Doctor realises that Jarra To is preparing to destroy the world -- and if this timeline collapses while the Axis is unstable, the Axis itself will crumble, spreading the madness throughout all time and space.

The facility’s lift then descends past the Doctor and his friends; though they don’t know it, this is Erimem on her way back to the basement. Assuming that the lift’s occupant was Jarra To, the Doctor waits for her to get clear and then takes the lift up to the laboratory on the top floor. It appears deserted, except for the TARDIS, and the Doctor sends Peri and Tog to look for Erimem while he shuts down the dimensional interface. However, he still believes that it was Jarra To in the lift -- and while he’s keeping an eye out in that direction, Jarra To emerges from the TARDIS, sneaks up behind him and knocks him out.

Tog and Peri return to the laboratory to find Erimem apparently crouched over the Doctor’s body. Convinced that this is the shape-shifting Jarra To, Peri nearly clubs her down with an iron bar, but Erimem quotes the passage they were reading from Alice in Wonderland. The real Jarra To steps out of hiding, somewhat disappointed that Peri wasn’t fooled into murdering her friend. Tog attacks Jarra To, and as they fight, the Doctor recovers and orders Peri to enter the TARDIS and use the emergency jump to move it out of Jarra To’s reach. Jarra To knocks Tog unconscious, but too late; the Doctor’s TARDIS dematerialises, and as the furious Jarra To turns on the Doctor, Erimem uses the ankh-key to unlock another door in the laboratory. The dead Time Lord’s TARDIS was here all along, and before Jarra To can stop her, Erimem enters and locks her out.

Jarra To and the Doctor are now stuck outside, and the Doctor informs Jarra To that he’s willing to die in the collapse of this timeline if this ends her insanity. All Time Lords are prepared to give their lives for the sanctity of the timelines, and if they often appear pompous, arrogant, or eccentric, it’s because they are always aware of the weight of their responsibility. This is why the dead Time Lord, Protok, refused to surrender his TARDIS even under torture, and why the Time Lords sealed off this timeline with him inside; he knew he was expendable, and so does the Doctor.

Furious, Jarra To shuts down the dimensional interface, postponing the destruction of her world. She’s convinced that Erimem will never work out how to pilot Protok’s TARDIS, and that it’s only a matter of time before she’s forced to open the door again; when she does, Jarra To intends to be waiting. First, she intends to throw the Doctor over the balcony to the Firebreed -- but as she prepares to do so, Tog revives and attacks her, knocking both her and himself over the edge of the balcony, and both plunge to their deaths amongst the magma fields and the Firebreed.