Whilst travelling alongside Halley's Comet, the TARDIS picks up an alien distress call from Earth. 

In the sewers below London's Fleet Street, a small gang of petty criminals – Griffiths (Brian Glover), Payne (James Beckett) and Russell (Terry Molloy), with their boss Lytton (Maurice Colbourne), ex-commander of the Dalek task force [Resurrection of the Daleks] – are apparently attempting to pull off a jewellery robbery. 

The Doctor and Peri arrive in London, the TARDIS materialising at 76 Totter's Lane [100,000 BC]. 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. The Doctor and Peri track the distress signal to a deserted house, but the Time Lord then realises that it is being relayed from elsewhere. They return to the TARDIS to follow the signal back to source, unaware that they are under surveillance by the two policeman-disguised androids (Mike Braben, Michael Jeffries) who also escaped from the Dalek spacecraft with Lytton.

The travellers arrive at a scrap metal yard and investigate a car inspection pit through which Lytton's gang has gained access to the sewers. They are confronted by the policemen, but the Doctor manages to disarm them. They then enter the sewers to investigate further. 

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 under cover 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 (David Banks) has agreed to transport him to Telos to meet the Cyber Controller (Michael Kilgarriff). 

On returning to the TARDIS – which is now 'disguised' as a pipe-organ – the Doctor, Peri and Russell are captured by the Cybermen, who have already found and entered the ship. 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 – a group of 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 (Jonathan David) and Bates (Michael Attwell), manage to escape. They intend to steal back their time ship which the Cybermen had captured, but unfortunately it requires three pilots. 

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. Peri is found and captured by Rost (Sarah Berger) and Varne (Sarah Greene), two of the Cryon race who were the original inhabitants of Telos, and taken to their base. The Doctor, meanwhile, is taken up to Cyber Control. 

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 [The Tenth Planet]. 

Lytton and Griffiths find their way to the Cryon base and from there make their way to hijack the time ship. 

The Doctor has been taken before the Cyber Controller, who apparently survived his earlier attempt to destroy him [The Tomb of the Cybermen]. The Controller orders that the Doctor be imprisoned in a sub-zero store room. There the Doctor meets Flast (Faith Brown), the Cryon leader. 

Flast 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 with this 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 locate Stratton and Bates on the Telosian surface 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. 

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 has 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 reunited. Peri explains the Cryons' plan to the Doctor and he attempts to rescue Lytton from Cyber Control. 

Lytton has already been partially cybernised. 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 kill each other in the confusion and the Doctor finally manages to destroy the Controller with a Cybergun. He is too late to save Lytton, however, who dies from his injuries. 

The Doctor and Peri leave Telos in the TARDIS just before a huge explosion, caused by the vastial reaching its ignition point, destroys the Cyber Control centre. The Doctor, safe in the TARDIS, ponders on how he could have so misjudged Lytton.

_{Synopsis from Doctor Who: The Sixth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}