As the Doctor and Peri argue over their next destination, the TARDIS is caught in a Kontron corridor, a time corridor in space -- a potentially disastrous situation. The Doctor struggles to realign the ship's co-ordinates to prevent it from exploding once it enters the tunnel, and he and Peri strap themselves to the console as the TARDIS approaches the tunnel and is nearly torn apart by waves of turbulence...

On the planet Karfel, three rebels are pursued through the Citadel's corridors, and are captured by guardoliers and androids. Aram is taken to the private vault of her leader, the Borad, who kills her for her betrayal; Gazak and Tyheer are taken to the Council chambers, where Tyheer pleads for his life, claiming that he is really a double-agent who led the Councillors to the rebels. But despite his pleas and Gazak's claim to be working for the good of Karfel, both are sentenced to the Timelash -- a permenant exile down a corridor through Time and Space.

Maylin Renis sets off to carry out his daily duties, while his daughter Vena and her future husband Mykros discuss what has happened to their planet. Vena is still innocent enough to believe that the Board is acting for the good of all Karfelons, but Mykros isn't convinced; since the Borad came to power their people have become divided and their former allies the Bandrils are posed to invade. The Borad never shows himself in person, only on the security monitors -- is this really a security measure or does he care more for his time experiments than his people?

Mykros follows Renis and slips into the power room behind him; only Renis is allowed here, and Mykros will face the ultimate penalty if caught. Fortunately this room isn't monitored, as the delta configuration rays damage communications equipment. Renis is in charge of transferring power through to the Borad's personal vault, but doesn't have the will to oppose him yet; if he did tamper with the transference the Borad would take his revenge on every Karfelon in the Citadel. Renis slots his official amulet into the power distribution panels, and reads his sealed orders for the day -- and is horrified to learn that he is to transfer all subsidiary power to the Borad's vault, including the power for the hospital, where his own wife is still on life-support systems after major surgery. He has no choice but to obey, but promises not to do anything to stop Mykros' attempts to bring down the Borad from within. However, he urges Mykros to be careful, for Vena's sake.

Renis and Mykros then leave the power room only to find an android waiting for them. Mykros is summoned to an emergency Council meeting and Renis is ordered to attend the Borad in person. In the Borad's vaults, Renis discovers that the Borad heard every word that passed between him and Mykros in the power room -- a listening device was embedded in the amulet and thus shielded from the delta configuration rays in the power room. The real Borad now reveals himself to Renis, and it's not the elderly man he knows from the security monitors but a repulsive creature apparently confined to an electronic wheelchair. The Borad shoots Renis with a time acceleration beam that ages him to dust within seconds, and orders an android to take the Maylin's amulet to the sycophantic Councillor Tekker.

Vena arrives for the Council meeting and is appalled to find that her father is dead -- ostensibly from a seizure of some sort -- and that Mykros, accused of treason, now faces the Timelash. Mykros begs her to stop the Borad by seizing the Maylin's amulet, and when Tekker approaches to "comfort" her, Vena snatches the amulet from around his neck and threatens to drop it into the Timelash. Tekker orders an android to retrieve it -- but it accidentally pushes Vena into the Timelash, and the amulet goes with her...

The TARDIS enters the corridor in one piece, but as the Doctor sets about stabilising the effect, the ghostlike form of Vena passes through the ship and is deflected onto a different course. A short-out on the console increases the turbulence as the TARDIS nears the source of the Kontron corridor. Its approach is detected on Karfel, where Tekker quickly realizes that a craft which can penetrate the Timelash is just what he needs to retrieve the amulet before the Borad exacts his revenge on every Karfelon in the Citadel. The Borad watches the TARDIS emerge from the Timelash, and looks forward to his reunion with the Doctor...

The Doctor recognizes the planet Karfel, which he visited in a former incarnation, but is puzzled; Karfel should be centuries away from the technology necessary to create the Timelash. He emerges to speak with Tekker, who evades the Doctor's questions and leads him and Peri to the hospitality suite. The android, still obeying its earlier instructions, snaps an amulet away from Peri's neck, and Tekker covers by claiming it was warning her away from poisonous plants in the wall baskets. He is then summoned away to answer a call from the Bandril ambassador; the Bandrils are threatening war since the Borad cut off grain shipments promised them by their treaty. Tekker is deliberately provocative and cuts the Bandril ambassador off in mid-transmission after it openly declares war on Karfel. He has total faith in the Borad, believing that he intends to destroy the Bandrils and lead the Karfelons in a war of conquest that will bring this sector to its knees.

Peri and the Doctor notice that there are no reflections from anything in the Citadel. A man slips in, hands Peri a mysterious message -- "Sezon at the Falchian Rocks" -- and rushes off. Peri stuffs the paper into her pocket as Tekker returns, claiming that he's arranged for Peri to tour the Citadel with Councillor Brunner while he speaks with the Doctor about the Timelash. In fact, Brunner is arranging with a guardolier to kill Peri and dispose of the body in the caves, where the Morlox will make a meal of her. As Peri leaves, Tekker asks the Doctor for his help in retrieving the amulet, but the Doctor doesn't believe Tekker's claim that Vena's fall into the Timelash was an accident and refuses to help. Until Tekker points out that Peri is now his prisoner...

Brunner guides Peri some distance away through the Citadel, and then departs, leaving her at the mercy of the murderous guardolier. Peri flings some of the poisonous plants in the guardolier's face and flees from a pursuing android, through a doorway into the caves outside the Citadel. But as she flees she drops the paper with the message she received earlier, and Brunner finds it. Peri, lost in the caves, follows an entrancing scent which leads her to the lair of a monstrous animal; rebels appear and gun down the Morlox, but are then surprised by the materialization of a burning android. The rebels drag Peri back to their base, where a moustached man prepares to kill her, accusing her of spying for the Borad. His associate Katz stops him, and when Peri claims to have come to Karfel with the Doctor, Katz shows her a photo of a woman whom Peri correctly identifies as the Doctor's former companion Jo Grant. Meanwhile, the Borad watches recordings of Peri's emergence from the TARDIS and decides that she has the spirit and intelligence he needs; she must be brought to him, unharmed...

The Doctor reluctantly returns to the TARDIS and calculates where Vena emerged from the Timelash. The Timelash co-ordinates are set for Earth in the year 1179 AD, but intersection with the TARDIS has deflected Vena to a small Scottish cottage in the year 1885, where a young teaching student named Herbert assumes that his attempts to contact the spirit world are responsible for her materialization. When the Doctor arrives to collect her, Herbert tries to exorcise him, but the Doctor calms him down and explain the situation to him and Vena. Vena reluctantly agrees to return with him, and Herbert, fascinated, asks to come along. The Doctor flatly refuses, and Herbert departs, aggrieved, while the Doctor picks up a hand mirror for future use on the mirrorless Karfel.

Peri shares a drink with the rebels, who explain that the Borad has driven Karfel to the brink of war with the Bandrils; and if the Bandrils use their bendalypse warheads they will kill every creature on Karfel with a central nervous system, leaving only the buildings and the Morlox. When Katz calls her moustached associate by name for the first time, Peri recognizes the name of Sezon, realizes that they're at the Falchian Rocks -- and realizes too late, as guardoliers burst in and take them all prisoner, that she dropped the note the rebel slipped her earlier. She is taken away separately; the Borad has other plans for her...

As an android marches the Doctor towards the Timelash, he uses Herbert's hand mirror to reflect light in its eyes, confusing it. The rebels take advantage of the distraction to attack their guards. Tekker and the nervous Councillor Kendron flee, while Mykros pushes Brunner into the Timelash. The rebels best the guardoliers and seal the doors, preparing for a siege. The Borad hears of the incident and sends an android to the Council room with a time web capable of disintegrating the doors.

The Doctor, meanwhile, has the others support him on a rope while he enters the Timelash to fetch Kontron crystals from the time corridor's walls. The Vortex attraction forces nearly pull him away from the walls into oblivion, but Herbert and Mykros rescue him at the risk of their own lives by climbing down the rope to pull him back out. The Doctor emerges with two Kontron crystals which he can use as tools to defeat the attacking androids -- and the Borad.

Tekker and Kendron go to speak to the Borad personally. Kendron is terrified of his leader's wrath, but Tekker assures him that he'll soon assuage the Borad's anger. But he does so by claiming that Kendron is a traitor who betrayed them all to the rebels; as Kendron protests his innocence, the real Borad -- the figure in the wheelchair -- shoots him in the back with his time acceleration beam. The elderly man thought by all to be the Borad is in fact an android used for public appearances. Tekker has bought himself some time. Meanwhile, Peri is dragged into the caves and chained up in a cavern near one of the ravenous Morlox creatures, and a canister of some sort is strapped to her.

The Doctor wires together the Kontron crystals with control units from the Timelash and builds a device capable of creating a ten-second "time break" -- he projects an image of himself ten seconds into the future, while his present state remains invisible. He uses the second crystal as a weapon; when connected to a guardolier's gun it will absorb the energy being used to break down the Council chamber's doors and refract it through Time back at the attackers. As he tries to set the time loop controls, Mykros sees a Bandril invasion force approaching on planetary radar. The Borad sees the approach as well, and is satisfied; soon the only living things on Karfel will be the Morloxes, and himself...

The guardoliers use the Borad's time web to age the doors into decay, and while the rebels fight them off, the Doctor uses his Kontron weapon to refract the time web's energy back at an attacking android. The android bursts into flame and vanishes backwards through Time -- to the tunnels about an hour beforehand, just as Katz and Sezon rescued Peri from the Morlox. The rebels defeat the attacking guardoliers, and the Doctor urges them to get to safety before the Borad sends more troops. During the fighting, part of the wall has shattered to reveal a mosaic of the Third Doctor, covered up by order of the Borad. The Doctor sets off to confront the Borad, and Herbert tags along, determined to be of help; instead of leaving, however, Mykros and Vena instead attempt to contact the Bandril fleet and convince them to call off their attack.

The Doctor advises Herbert to wait outside while he confronts the Borad, as this could be terribly dangerous. Herbert climbs into a gallery overlooking the Borad's vault and watches as the Doctor enters and confronts Tekker -- and the real Borad, who is revealed to be a mutant, half-Karfelon and half-Morlox. The Doctor eventually recognizes him as an old acquaintance; a scientist named Megelen, whom the Doctor had befriended on his first visit but then turned in to the Council when he discovered Megelen was engaged in unethical experiments on the Morloxes. Megelen continued with his experiments until an accident occurred; he was attacked by a Morlox after accidentally spraying himself with the chemical Mustakozene-80, and the chemical caused his tissue and that of the Morlox to amalgamate into a single living organism. Now, as the Borad, he has greater strength and intelligence and a longer lifespan than an ordinary Karfelon -- and he has provoked war with the Bandrils to wipe out the ordinary people of Karfel and repopulate the planet with his own kind. Tekker is horrified to discover the Borad's true intentions and vows to stop him, and the Borad almost casually disposes of him with his time acceleration ray and continues his explanation. Peri has been chained up near a Morlox with a canister of Mustakozene-80; the Borad intends to transform her into his mate.

The Doctor, furious, switches on his time-break crystal, and when the Borad tires of him and tries to shoot him with his time acceleration ray, the beam goes right through the Doctor's intangible image. The real Doctor searches the room for the release mechanism which will free Peri but is unable to locate it, and eventually shifts himself back into real time to demand that the Borad release her. The Borad refuses and fires at the Doctor, ignoring his warnings -- and the Doctor's Kontron crystal absorbs the energy and refracts it back at the Borad, killing him. The Doctor orders the sickened Herbert to rescue Peri, and eventually finds the release mechanism on the Borad's chair. Unfortunately it releases both Peri and the Morlox at the same time, but Herbert is able to drive off the Morlox with a flaming torch, and he and Peri flee to safety.

The Doctor finds Herbert removing the Mustakozene-80 canister from Peri, and takes them back to the Council chamber, hoping to stop the Bandrils from invading. But he's too late; the Bandrils have refused to accept any further transmissions from Karfel, and by the time the Doctor arrives they have already fired their bendalypse warhead. The Doctor contacts them, identifying himself as President of the High Council of Gallifrey, and they agree to abort the missile only if they are shown proof that the Borad is dead. The Karfelons are unable to present that proof, so the Bandrils cut transmission.

The Doctor rushes back to the TARDIS, ordering Peri to stay with the others; she ignores his instructions and follows him, but he tries to physically bundle her out of the TARDIS, telling her that he must try to deactivate the missile alone. She eventually concedes and goes, but as the Doctor dematerializes, he discovers that Herbert, still seeking to be in on the thick of things, slipped on board while the Doctor was arguing with Peri. Herbert is shocked when the Doctor admits that he was lying to Peri to save her life; the only way to stop the warhead is to materialize the TARDIS in its path, sacrificing his own life to detonate the missile prematurely before it reaches the planet.

Peri and the Karfelons see the missile's explosion and assume that the Doctor was able to talk sense into the Bandrils -- until the Bandril ambassadors contacts them with abject apologies and offers to send a diplomatic party to honour the Doctor's sacrifice. Peri stumbles away from the others to come to terms with the Doctor's death -- and is unexpectedly captured by the Borad, who threatens to kill her unless the Karfelons capture the approaching Bandril diplomatic party and hold them hostage. The Doctor and Herbert then arrive, and learn that the Borad they saw die was only a clone, part of another genetic experiment. The Doctor deliberately provokes the Borad by mocking his gruesome appearance, having realized that the Borad hid away from the people of Karfel because of self-loathing and fear of rejection. He picks up a chair and smashes the mosaic of his earlier self to reveal a mirror bricked up behind it; when the Borad sees himself reflected in it he staggers back, traumatised. As Katz drags Peri to safety, the Doctor activates the Timelash and pushes the Borad in.