The Second Doctor and Jamie McCrimmon land the TARDIS on board Space Station Camera in the Third Zone, on a mission for the Time Lords, who have also installed a teleport control on the TARDIS that grants them dual control for the occasion. The Doctor explains to Jamie that the station is a research facility and they are here to have a discreet word with Dastari, the Head of Projects. The TARDIS materialises in the station kitchen, where they meet Shockeye, the station cook. Shockeye is an Androgum, a member of a primitive, emotionally and ethically bestial humanoid race which acts as the station's workforce, and is confrontational until the Doctor reveals he is a Time Lord. Suddenly deferential, Shockeye eyes Jamie hungrily and offers to buy him from the Doctor as the main ingredient for a meal. The Doctor, shocked, refuses, and takes Jamie away to see Dastari. As they leave, however, they hear the sound of the TARDIS dematerialising. This is observed by Chessene, an Androgum technologically augmented to mega-genius levels. Chessene has plans of her own, involving someone named Stike who will be arriving in force soon, once Shockeye's poisoned meal to the scientists takes effect. She has also taken possession of the Kartz-Reimer module.
The Doctor speaks to Dastari in his office, telling him that the Time Lords want the time experiments of Kartz and Reimer stopped. The Time Lords have an official policy of neutrality, and so have sent the exiled Doctor to maintain deniability. Dastari introduces Chessene, but the Doctor is sceptical as to whether such augmentation can change Chessene's essential Androgum nature, and he considers such tampering dangerous. Meanwhile, three Sontaran battlecruisers appear near the station, on an intercept course. Before the station's defences can be activated, Chessene incapacitates the technician on post and opens the docking bays. Back in the office, the Doctor warns that the distortions from the Kartz-Reimer experiments are on the verge of threatening the fabric of time, but Dastari refuses to order them to cease, accusing the Time Lords of not wanting another race to discover the secrets of time travel. As the argument grows more heated, Dastari grows faint and falls into a drugged stupor. Energy weapons fire begins to sound in the corridors and the Doctor orders Jamie to run as a Sontaran levels a gun at the Doctor.
Somewhere else, the Sixth Doctor and Peri Brown are on a peaceful fishing trip. When they return to the TARDIS, Peri is startled as the Sixth Doctor sways and collapses — just as, back on the station, Jamie spies the Second Doctor in a glass chamber, writhing in agony as a Sontaran manipulates controls. In his TARDIS, the Sixth Doctor awakens, somehow having had a vision of himself as his second incarnation being put to death. He realizes that this is impossible, since he is still alive, but he is also concerned that he may have died in the past and only exists now as a temporal anomaly. He decides to go and consult his old friend Dastari to see if he can enlist his help.
The TARDIS materializes on the station, but everything is dark, and the smell of decay and death is everywhere. The station computer demands the Doctor leave, and when he refuses, tries to kill him and Peri by depressurising the passageway. The Doctor, however, manages to open a hatch and drag his unconscious companion through to another section. In Dastari's office, the Doctor discovers the scientist's day journal and the Time Lords' objections to the Kartz-Reimer experiments, but refuses to believe his people are responsible for the massacre. Peri suggests someone is trying to frame the Time Lords and drive a wedge between them and the Third Zone governments. They leave the office to enter the service ducts, work their way to the control centre and attempt to deactivate the computer before it succeeds in killing them.
On Earth, Chessene, Shockeye and a Sontaran, Major Varl, take possession of a Spanish hacienda by killing its aged owner, Doña Arana. Varl sets up a homing beacon for the Sontaran ship, while Chessene absorbs the knowledge of the old woman's mind, discovering that they are in Andalucia, just outside the city of Seville. Varl announces that Group Marshal Stike of the Ninth Sontaran Battle Fleet is in descent orbit. Meanwhile, two people, Oscar Botcherby and Anita, are approaching the grounds. Oscar, an ex-English stage actor who is managing a restaurant in the city, is here to catch moths, armed with a net and a cyanide killing jar in his backpack. He and Anita see the Sontaran ship zoom overhead, and observe through binoculars Dastari and another Sontaran carrying an unconscious Second Doctor towards the hacienda. Anita pulls Oscar along, thinking that they are victims of an aeroplane crash and need help.
Down in the bowels of the station, the Sixth Doctor tries to disconnect the main circuit. Suddenly, Peri is attacked by a humanoid in rags, and when her cries distract the Doctor, he is hit by a gas trap and falls unconscious, becoming entangled in the wires.
Peri knocks out her attacker and frees the Sixth Doctor, who saved himself by shutting off his respiratory passages. He disconnects the computer's main circuit, and the two find that Peri's attacker was a half-delirious Jamie, who has been hiding all the while. Jamie moans that "they" killed the Doctor, and under hypnosis, tells the Sixth Doctor what has transpired, giving a description that the Doctor recognizes as the Sontarans. Returning to the office to examine the station records, the Doctor suddenly sees Peri in the glass tube, writhing in pain. As he frantically works the controls to free her, the person in the tube changes from Peri to Dastari to the Second Doctor and even to himself. When Jamie and Peri return to the office, the Sixth Doctor explains that what Jamie saw was an illusion designed to make people believe the Doctor was dead and not investigate further (the animator had been left on and captured Peri's image), which means the Second Doctor is being held captive somewhere. The Sixth Doctor theorises that the Sontarans kidnapped Dastari as well because Dastari is the only biogeneticist in the galaxy who could isolate the symbiotic nuclei of a Time Lord that gives them the molecular stability to travel through time. If given time travel, the Sontarans will be unstoppable. The Sixth Doctor decides to put himself into a telepathic trance to try and determine where his past incarnation is being held. He awakens having heard the sound of the Santa Maria, the largest of the 25 bells at the Great Cathedral of Seville.
In the cellar of the hacienda, Dastari and Chessene set up equipment, keeping the Second Doctor drugged and passive. Dastari questions why they have come to Earth, and Chessene explains that it is conveniently situated for an attack Stike wishes to make on the Madillon Cluster against the Rutan Host, and that Shockeye also wanted to taste the flesh of humans. Dastari heaps scorn on Shockeye's primitive urges, and urges Chessene to remember that she is beyond those, now. The TARDIS materialises on the grounds near the hacienda, and Oscar approaches it as the TARDIS crew emerge, thinking it is a real police box and that the Doctor and his companions are plain-clothes police officers. Taking advantage of the mistake, the Doctor asks that Oscar lead him to the hacienda.
Dastari reveals his plan to dissect the Second Doctor's cell structure to isolate his symbiotic nuclei and give them to Chessene. The Second Doctor calls him mad, and protests that her barbaric Androgum nature, coupled with the ability to time travel, will mean that there will be no limit to her evil. The Sixth Doctor asks Peri to create a distraction at the front door of the hacienda while he and Jamie make their way into the cellar via a passage in the nearby ice house. Peri calls out, interrupting Dastari's operation. She poses as a lost American student, but Chessene is suspicious, having read thoughts of the Doctor in her mind. Chessene gets Shockeye to bring the Second Doctor, strapped into a wheelchair, through the hall, to see if Peri reacts. She does not, as she has never seen the Second Doctor before. Peri makes her excuses and leaves, but Shockeye chases her anyway, eager for a meal.
Meanwhile, the Sixth Doctor and Jamie are in the cellar, where the Doctor examines the Kartz-Reimer module, a prototype time machine modelled on Time Lord technology. He explains to Jamie that once the briode nebuliser of the module is primed with his symbiotic nuclei — the Rassilon Imprimatur — it will be safe for anyone to use. Unfortunately, the Sontarans have heard him. Outside, Shockeye also catches up to Peri.
Shockeye knocks Peri out and brings her back to the hacienda kitchen. In the cellar, Stike threatens to kill Jamie unless the Sixth Doctor gets into the module and primes it with his symbiotic print, and the Doctor does so. Stike is about to execute Jamie anyway, but Jamie stabs Stike's leg with a concealed knife, and the Doctor and he run off upstairs, where they find the Second Doctor. Before they can release the Second Doctor and escape the hacienda, however, Shockeye shows up with the unconscious Peri. The Second Doctor feigns unconsciousness while the others hide.
While the Sixth Doctor and Jamie watch from their hiding place, they hear Chessene voice her concern that now that a second Time Lord is involved, the other Time Lords will be arriving as well. However, she has a contingency plan. She asks Dastari to implant the Second Doctor with some of Shockeye's genetic material, turning the Doctor into an Androgum and under her thrall, following which they will eliminate the Sontarans. However, Dastari and Chessene are unaware that the module is now primed, and that, outside, Stike is preparing to leave in it once Sontaran High Command has been notified and leave no one alive when he does so. Stike orders Varl to set the Sontaran battlecraft's self-destruct mechanism.
Interrupting Shockeye as he is about to slaughter Peri, Chessene gets him to bring the Second Doctor to the cellar. Once there, she stuns Shockeye so that Dastari can remove his genetic material. The Sixth Doctor revives Peri in the kitchen and ushers her and Jamie away. The Sixth Doctor tells them that what he revealed about the Imprimatur in the cellar was not strictly true — he had heard Stike approaching and the speech was for the Sontaran's benefit. The machine worked for the Doctor, but will not for them because the Doctor has taken the briode nebuliser.
Dastari has implanted the Second Doctor with a 50 percent Androgum inheritance, and when Shockeye wakes in a rage, he finds a kindred spirit in the transformed Doctor. They decide to go into the town to sample the local cuisine. In the meantime, Dastari lures the Sontarans into the cellar, where Chessene attacks them with two canisters of coronic acid. Varl is killed, but Stike, though wounded, manages to escape. He tries to use the module, but without the nebuliser, it severely burns him instead. Stike staggers towards his battlecraft, forgetting about the self-destruct. The ship explodes, taking him with it.
The Sixth Doctor, Peri and Jamie follow the Second Doctor and Shockeye into Seville, hoping to cure him before the change becomes complete and affects the Sixth Doctor as well. Dastari and Chessene are also seeking the two of them, knowing that unless the Second Doctor undergoes a second, stabilizing operation, he will eventually reject the Androgum transfusion. The Second Doctor and Shockeye go to Oscar's restaurant, ordering gargantuan amounts of food. When Oscar demands that they pay, Shockeye fatally stabs Oscar, just as the Sixth Doctor and the others arrive. Shockeye leaves the Second Doctor behind, who slowly reverts back to normal. As all of them leave the restaurant and the distraught Anita, however, Chessene and Dastari appear, taking them back to the hacienda at gunpoint.
Chessene and Dastari find the nebuliser on the module missing, and the Sixth Doctor tells them how he primed the machine for Stike. To test the truth of the Doctor's claim, they replace the nebuliser and send Peri on a trip with the module, and she survives. Chessene gives permission for Shockeye to eat Jamie, and the Androgum takes him up to the kitchen. Left alone for the moment, the Sixth Doctor smugly confirms the Second's suspicions — the nebuliser is sabotaged, with a thin interface layer so it would only work once for Peri. Flipping the table over on which the key to their chains rests, the Doctors retrieve the key. The Sixth Doctor frees himself first, and runs up to save Jamie. He encounters Shockeye in the kitchen, and the Androgum wounds him with a knife. Shockeye pursues him through the grounds, but the Sixth Doctor finds Oscar's pack and his cyanide killing jar. The Doctor ambushes Shockeye, covering his head with Oscar's butterfly net and pressing the cyanide-soaked cotton wool to his face, killing him.
The sight of the Time Lord's blood on the ground is too much for Chessene, who falls to her knees and starts licking it, to Dastari's disgust. He realizes that no matter how augmented she may be, Chessene will always be an Androgum, and decides to free the Second Doctor and his companions. When Chessene sees this, she shoots and kills Dastari. She tries to shoot the Second Doctor and Peri as well, but Jamie throws a knife at her wrist, making her drop the gun. Chessene goes into the module, hoping to escape, but the module explodes, molecularly disintegrating her and turning her back into a common Androgum in death.
The Second Doctor uses a Stattenheim remote control — which the Sixth Doctor covets — to summon his TARDIS. He and Jamie say their goodbyes and leave. As the Sixth Doctor and Peri make their way back to their own TARDIS, the Doctor tells her that from now on, it will be a healthy vegetarian diet for both of them.
Doctor Colin Baker (Sixth Doctor)
Patrick Troughton (Second Doctor)
Companion Nicola Bryant (Peri Brown)
Frazer Hines (Jamie McCrimmon)
John Stratton — Shockeye
Jacqueline Pearce — Chessene
Laurence Payne — Dastari
Nicholas Fawcett — Technician
James Saxon — Oscar
Carmen Gómez — Anita
Aimée Delamain — Doña Arana
Clinton Greyn — Stike
Tim Raynham — Varl
Writer Robert Holmes
Director Peter Moffatt
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6W
Series Season 22
Length 3 episodes, 45 minutes each
Originally broadcast February 16–March 2, 1985