In the London sewer system, a worker vanishes and another is beaten to death.
The Doctor is repairing the chameleon circuitry in the TARDIS's roundels, using his new sonic lance. He ponders why he has not worked on this before. Peri questions his new energy levels; he reassures her he is stable and would never hurt her.
Lytton is organising what he claims is a £10 million diamond heist on the Bank of England. He explains the plan: his merry band of four shall go into the sewers, and use plastic explosives to blow a hole in the wall of the vault, escaping with the diamonds, and no one (in theory) should get hurt. Down they go into the sewers, with Payne agreeing to stand on lookout by the manhole. As the others move away, no one notices the tall, black figure silently advancing behind Payne...
The Doctor says he is taking Peri somewhere nice and peaceful, to treat her after the awful time they both had on Jaconda. After a very difficult trip through the Time Vortex, he shows Halley's Comet to her, inadvertently revealing that he plans to take her to Earth. It is soon clear that being so close to the comet upsets her (as does, undoubtedly, the fact that they nearly crash into it), so he steers away from it. The TARDIS then picks up a distress signal coming from London, in 1985; they both agree that they have to investigate this.
The TARDIS lands in 76 Totter's Lane, London, a scrapyard that the Doctor finds oddly familiar. As he and Peri begin to leave the scrapyard the chameleon circuit turns the time machine into a stove with an attractive (or cloying) floral pattern (much to Peri's mirth). The Doctor, slightly defensive, says that the TARDIS is slightly out of practice when it comes to choosing new forms. (They both fail to notice two policemen, who are walking past them.) As the pair move through the streets, the Doctor scanning for this signal, Peri reveals how worried she is for him: his memory is in pieces, and he keeps calling her the names of his previous companions. He assures her he is fine. After tracing the signal to an abandoned warehouse that does not contain anyone; he remarks how foolish he was for not realising what has happened. After dashing back to the scrapyard, they eventually find a door in the TARDIS's new form and take off.
Meanwhile, Lytton's group are not faring well in the sewer: Russell has cold feet, and Griffith is doing all of the wall-demolishing single-handedly (much to his annoyance). Lytton does not seem to be noticing these things, and seems almost to be waiting for someone.
Onboard the TARDIS, the Doctor explains that the alien has put relays around the city, making it hard for them to trace his signal (and thus help him). Peri points out a vital clue: such an extraterrestrial would surely leave a time trace; the Doctor starts tracking down that very thing.
The TARDIS then lands, disguised as a pipe organ, in the garage containing the manhole Lytton's crew have descended. There, the two policemen seen earlier accost them, but the Doctor (unseen) knocks one of them out in the sewer, and Peri handcuffs the other to a railing and takes his gun. They then descend the manhole.
In the sewer, Lytton's trio discover a tall, black figure advancing towards them. Although Lytton insists that all is fine, Griffith panics and shoots his (previously unseen) gun at the tall figure; prompting Lytton to take out his own firearm and threaten Griffith, in order to stop him firing at the figure. Suddenly, the wall behind them slides open and an entire army of silver giants is revealed. Then Lytton offers their Leader his weapon, saying that he offers his life to the Cybermen. The Cyber Leader effortlessly crushes Lytton's gun, eliciting a scream from Griffith...
Lytton's two policemen comrades - as well as the two sewer workers we saw at the beginning - are being converted into Cybermen. Lytton manages to talk his way out of the same procedure, explaining that he detected the Cybermen's transmissions and deliberately contacted them, bringing along humans for them to convert as a sign of goodwill. He identifies himself as a warrior mercenary from Riften V and points out that he could easily have alerted Earth authorities to the Cybermen's presence but chose not to. The Cyber Leader accepts the logic of his argument and decides to report to the Controller on Telos.
On Telos, a work party of slaves plants explosives in the ground. Three of them make a break for it, but one is killed and the decapitated Cyber-head, which they require for the next stage of the escape, is destroyed. The two survivors, Bates and Stratton, hide nearby, but without a third pilot and a Cyber-head, they're still as good as prisoners. The other slaves' spirits have been completely crushed; nobody else has tried to escape. In Cyber Control, the Controller receives a report of the escape attempt, and decides to analyse Bates and Stratton's behaviour as they attempt to survive and escape.
The Doctor and Peri are captured by Russell, who frisks the Doctor and finds Payne's gun. The Doctor manages to surprise and overpower Russell, who eventually admits that he's an undercover policeman who infiltrated Lytton's gang to find out who he was. After a raid on an electronics warehouse -- which the Doctor and Peri realize supplied Lytton with the parts he needed for his intergalactic transmitter -- the police heard Lytton's name whispered on the streets, but could find no records of his existence at all. It was as if he'd just arrived from another planet. The Doctor warns Russell that this is exactly what he did -- and he's a ruthless, professional killer...
Bates and Stratton use their mining tools to destroy and decapitate a Cyberman sent out to recapture them. Bates intends to clean out the head so Stratton can use it as a disguise; as prisoner and escort they stand a better chance of getting into Cyber Control. But the destruction of the scout is detected, and the Controller decides that Bates and Stratton are too resourceful and must be destroyed.
Back on Earth, the Cybermen detect temporal distortion nearby, and send scouts to investigate. The Doctor, Peri and Russell encounter one, and the Doctor destroys it by plunging his sonic lance into its chest unit. The Cybermen detect this, and the Leader decides to close down this base and send the partially converted humans to their mothership. The Leader himself takes a squad out to investigate the scout's destruction, and when they find an alien artefact was responsible Lytton soon guesses who the "alien" is. He's surprised to learn that the Cybermen already know of the Doctor. The Cyber Leader decides to alter his plans and capture the Doctor and his TARDIS.
The Doctor, Peri and Russell emerge from the sewers, closely followed by the Cybermen. But the Doctor has accidentally left the TARDIS doors open and Cybermen have already entered the ship. Russell destroys one by shooting it through the weak point in its mouth panel, and shoots another with the first Cyberman's gun. But before Peri can shut the doors the Cyber Leader and his patrol arrive, and while Russell is distracted a third Cyberman emerges from the corridors and strikes him upon his neck, killing him instantly. Peri approaches Russell and the Cybermen then close in on Peri...
The Doctor threatens to destroy the TARDIS unless the Cyber Leader agrees to spare Peri's life. He does so, giving the word of the Cyber Controller that she will not be harmed -- and the Doctor realizes that, by implication, not only did the Controller survive their last meeting but these Cybermen have somehow travelled through Time. He sets the coordinates for Telos, and he, Peri, Griffiths and Lytton are locked up in a nearby storeroom. Lytton returns the Doctor's sonic lance so he can sabotage the navigational controls and shift the TARDIS slightly off course, and reveals that the Cybermen haven't developed their own theories of Time travel; they simply stole a ship which was forced down on Telos for repairs. The Doctor, attempting to explain the history of the Cybermen to Griffiths and Peri, is forced to admit that their home world Mondas was destroyed while attacking Earth -- in 1986, which in their terms is next year. The Doctor assures them that Earth survived with minimal damage; the surviving Cybermen evacuated to Telos, wiped out the indigenous Cryons and transformed their refrigerated cities into cryogenic tombs in which to hibernate and recover their strength. Bates and Stratton continue to approach Cyber Control despite Stratton's conviction that the plan will never work. The reactivation of dormant Cybermen is halted when too many are found damaged or dead; some are going rogue in the tombs and destroying everything they encounter.
The Doctor is forced to switch off the distress call he'd surreptitiously activated, but thanks to his earlier sabotage the TARDIS (in the form of a set of iron gates) materializes in the tombs rather than in Cyber Control. While the Cyber Leader reports for further instructions, the Doctor notices a stench of decay in the air -- and realizes that Lytton knows more about it than he's saying. A rogue Cyberman suddenly bursts out of a tomb and attacks them, and in the confusion Peri, Lytton and Griffiths escape. Peri, separated from the others, is attacked by yet another rogue Cyberman -- and is rescued by two Cryons...
Griffiths and Lytton hide in the tunnels outside the tombs, where they are contacted by a Cryon named Threst -- who welcomes Lytton by name. Lytton admits that he's been working for the Cryons all along; it was they who picked up his distress call from Earth, and on their behalf he intends to steal the Cybermen's time machine. Since the Cryons can only survive in sub-zero temperatures they will be unable to help, and Lytton thus brought Griffiths along to act as his bodyguard, in return for which the Cryons will pay him the equivalent of two million British pounds in uncut diamonds. Griffiths is reluctant to risk his life, but Lytton points out that his only two alternatives if captured are death -- or conversion into a Cyberman.
The Doctor is locked up in a storeroom with a Cryon prisoner, Flast, and upon learning that some Cryons survived the Cybermen's attempt at genocide he also realizes that they must be responsible for the damage to the Cybermen in the tombs. He's less pleased by Flast's revelation of the Cybermen's plans -- since they stole their time machine they don't fully understand the principles of Time, and intend to change history by preventing Mondas from being destroyed...
Lytton and Griffiths emerge onto the surface of Telos, where Bates and Stratton confront them. Griffiths is shocked to learn that Bates and Stratton are partially cybernetic; they were sent to the work parties when the conversion process failed. Lytton points out that the time vessel requires a crew of three and suggests that they join forces. Meanwhile, Peri is held in the Cryon base by Rost and Varne, who are unable to help her rescue the Doctor as they would perish in the heat of Cyber Control. They admit that Lytton is working for them to prevent the Cybermen from leaving Telos -- upon abandoning the planet the Cybermen intend to destroy it to observe the effect on its atmosphere.
Flast explains to the Doctor that the Cybermen intend to divert the course of Halley's Comet, causing it to collide with Earth. The Doctor suddenly realizes that the Time Lords have once again manipulated him into this situation so he can clean it up for them. Flast points out a potential weapon; the storeroom contains canisters of vastial, an unstable mineral which explodes upon reaching fifteen degrees above zero, and she's managed to open one. The Doctor uses his sonic lance to pick the lock of the storeroom door, and uses a small amount of vastial to destroy the guard outside. Flast takes the sonic lance, turns it on and buries it in the open canister of vastial, hoping to spark an explosion which will destroy Cyber Control. She is unable to leave the sub-zero storeroom but urges the Doctor to escape without her.
Lytton and his companions enter Cyber Control, but as Lytton is guarding their backs he is attacked and overpowered by Cybermen and the others have no choice but to carry on without him. Lytton is taken back to the control room and tortured, and when he refuses to speak he is taken to be converted into a Cyberman. Rost and Varne learn of Lytton's capture while taking Peri back to the TARDIS.
The Doctor returns to the TARDIS, where he finds two Cybermen on guard and is reunited with Peri. Rost and Varne help him break into a tomb which they have already sabotaged, and the Doctor activates the distress call in the dead Cyberman inside, luring the two guards away from the TARDIS and into a trap. In the ensuing battle, Varne is killed but both Cybermen are destroyed. As the Doctor prepares to leave, Peri insists that they rescue Lytton first, and the Doctor, who was fully prepared to leave Lytton to his fate, is startled to learn that he was working for the Cryons all along. He agrees to see what he can do.
Bates, Griffiths and Stratton finally reach the landing pad, but just as they're within sight of their goal Bates is killed by an electrified door -- which opens to reveal a Cyberman who guns down Griffiths and Stratton. Meanwhile, the Cybermen detect the Doctor's escape and question Flast; when she refuses to speak they fling her into the corridor, where her body boils away in the heat. As the Cybermen begin checking the vastial stores, the Cyber Controller learns that the TARDIS has been moved and returns to the control room. The sabotaged vastial container, hidden in the back of the storeroom, has begun to steam...
The TARDIS, once again in the form of a police box, materializes in the control room. The Doctor emerges to find Lytton partially converted, and as he tries to free him from the processing machine Lytton, drugged and partially converted, begs the Doctor to kill him. The Cyber Controller arrives, having guessed that the Doctor's emotional weaknesses would draw him back to rescue his friend. As the Controller approaches, however, Lytton attacks him, puncturing his hydraulic valves with the knife the Doctor was using to pry him free from the processing machines. The Controller strikes back, snapping Lytton's neck and killing him, while the Doctor grabs the Controller's gun and shoots the Cyber Leader, who staggers back into his Lieutenant, causing him to accidentally fire his gun at point-blank range, killing them both. The Doctor then shoots the Cyber Controller, destroying him once and for all. Peri emerges from the TARDIS and practically drags the Doctor away from Lytton's body.
As the TARDIS dematerializes, the sonic lance finally heats the vastial to ignition point, and the resulting chain reaction destroys all Cyber Control and the stolen time machine as well. The Earth is safe and the web of Time has been preserved... but at a great personal cost, as the Doctor blames himself for misjudging and failing to save Lytton.
Doctor Colin Baker (Sixth Doctor)
Companion Nicola Bryant (Peri Brown)
Maurice Colbourne — Lytton
Brian Glover — Griffiths
Terry Molloy — Russell
James Beckett — Payne
Jonathan David — Stratton
Michael Attwell — Bates
Stephen Churchett — Bill
Stephen Wale — David
Sarah Berger — Rost
Esther Freud — Threst
Sarah Greene — Varne
Faith Brown — Flast
David Banks — Cyber Leader
Michael Kilgarriff — Cyber Controller
Brian Orrell— Cyber Lieutenant
John Ainley — Cyberman
Writer "Paula Moore" (Paula Woolsey)
Director Matthew Robinson
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6T
Series Season 22
Length 2 episodes, 45 minutes each
Originally broadcast January 5–January 12, 1985