The Doctor and Peri

Whilst travelling alongside Halley’s Comet, the TARDIS picks up an alien distress call from Earth. The Doctor and his travelling companion Peri arrive in London, the TARDIS materialising once again at 76 Totter’s Lane (see "An Unearthly Child"). As The Doctor had been trying to repair the chameleon circuits, the TARDIS takes a few seconds to adjust and then transforms into a large, decorated cupboard. On leaving the TARDIS, The Doctor and Peri track the distress signal to a deserted house, but The Doctor realises that it is being relayed from elsewhere. They therefore return to the TARDIS to follow the signal back to its source, unaware that they are under surveillance by the two policeman.

Meanwhile in the sewers below London’s Fleet Street, a small gang of petty criminals - Griffiths, Payne and Russell, with their boss Lytton, ex-commander of the Dalek task force (see "Resurrection of the Daleks") - are apparently attempting to pull off a jewellery robbery.

Back in the TARDIS, The Doctor manages to locate the true source of the distress signal and so The Doctor and Peri arrive at a scrap metal yard - The TARDIS now disguised as a pipe-organ - and they investigate a car inspection pit through which Lytton’s earlier gang used to gain access to the sewers. They are confronted by the policemen (who are actually disguised androids who also escaped from the Dalek spacecraft with Lytton), but The Doctor manages to disarm them. The Doctor and Peri then enter the sewers to investigate further.

"This Looks Familiar…"

Lytton has contrived to get his unwitting gang into the sewers to bring them to the Cybermen, who have established a secret base there. Payne is killed by a Cyber Scout in the tunnels and Lytton and Griffiths are captured by the Cybermen. Russell - in reality an undercover policeman - runs back through the sewers and encounters The Doctor and Peri. Lytton has meanwhile offered his help to the Cybermen and the Cyber Leader has agreed to transport him to the planet Telos to meet the Cyber Controller. On returning to the TARDIS, The Doctor, Peri and Russell are captured by the Cybermen, who have already found and entered the time machine. Russell is killed and the Cyber Leader forces The Doctor to transport them all to Telos.

Telos is being mined by the Cybermen using slave labour - partially cybernised humans who were found to be unsuitable for complete conversion. Three of these workers make a bid for freedom, but only two, Stratton and Bates, manage to escape. They intend to steal back their time ship which the Cybermen had captured, but unfortunately it requires three pilots.

On Telos

Thanks to The Doctor interfering with the guidance systems, the TARDIS materialises, in the form of a gateway, in the lower levels of the tombs on Telos, slightly off course. On emerging, the occupants are attacked by a rogue Cyberman and, in the confusion, Peri, Lytton and Griffiths manage to escape. While Lytton and Griffiths find their way to the Cryon base and from there make their way to hijack the time ship, Peri is found and captured by Rost and Varne, two of the Cryon race who were the original inhabitants of Telos, and taken to their base. It is there that Peri discovers that Lytton is in fact working for the Cryons to try to thwart the Cybermen’s plan of knocking Halley’s Comet off course in 1985 and crashing it into the Earth, thus preventing the destruction of the Cybermen’s original planet Mondas in 1986 (see "The Tenth Planet").

The Doctor meanwhile has been taken to Cyber Control and there he meets the Cyber Controller, who apparently survived his earlier attempt to destroy him (see "The Tomb of the Cybermen"). The Controller orders that The Doctor be imprisoned in a sub-zero store room. There The Doctor meets Flast, the Cryon leader, who tells The Doctor of the Cybermen’s plan and he guesses that he has been manoeuvred by the Time Lords into being their agent to prevent the alteration of history. Flast explains to The Doctor that they are being held in a refrigerated chamber which also contains many crates of vastial, a mineral found in the coldest areas of Telos which becomes extremely volatile in temperatures above ten degrees and self-ignites at fifteen degrees. The Cybermen intend to destroy Telos when they leave, as an experiment to see what it will do to the planet’s atmosphere. The Doctor uses a small amount of vastial to despatch their guard and then leaves Flast with a sonic lance which should have sufficient power remaining to ignite the rest of the mineral. Flast activates the lance and hides it in a box of vastial.

Lytton and Griffiths meanwhile manage to locate Stratton and Bates, on the surface of the planet. and the four of them join forces to capture the time ship. They are almost there when Lytton is re-captured by the Cybermen. He is tortured until he reveals his plan and the Cybermen then set up an ambush at the time ship. Bates is electrocuted when he touches the door to the ship, and Griffiths and Stratton are gunned down by Cybermen waiting inside.

Bates

Discovering that The Doctor has escaped from the store room, the Cybermen investigate. They kill Flast by dragging her out of the cold storage room into the warm corridor - Cryons being able to exist only in sub-zero temperatures - and then start to check the vastial in case The Doctor had interfered with it.

With arrangements made for Lytton to steal the time ship, the Cryons take Peri to the TARDIS with the intention of forcing her to move it, leaving the Cybermen with no means of carrying out their plan. The Cybermen guarding the TARDIS are killed and The Doctor and Peri are reunited. Peri explains the Cryons’ plan to The Doctor who then decides to try and rescue Lytton from Cyber Control.

Lytton though has already been partially cybernised. On discovering this The Doctor attempts to remove him from the machinery, but is interrupted by the arrival of the Cyber Controller. The weakened Lytton manages to stab the Controller, who rains down blows upon him before turning his attention to The Doctor. Two Cybermen arrive but they kill each other in the confusion and The Doctor finally manages to destroy the Cyber Controller with a Cybergun. He is too late though to save Lytton, who dies from his injuries.

With the Cybermen’s plans ruined The Doctor and Peri leave Telos in the TARDIS just before a huge explosion, caused by the rigged box of vastial detonating when it reachs its ignition point. This sets off a chain reaction of explosions that obliterates Cyber Control and the tombs. The Doctor, safe in the TARDIS, feels that things had not gone very well and, despite reassurance from Peri, ponders on how he could have misjudged Lytton so badly.