Lieutenant Scott and his men climb up a bleak hillside, escorting Professor Kyle to a camp set up by Scott's team. The scanning equipment at the campsite is directed at a cave and set up to detect mammalian life forms. Kyle had been part of a scientific expedition investigating the caves for fossil remains, but the expedition has all but disappeared. The lack of readings suggests they are all dead. Kyle's knowledge of the tunnels is necessary to direct Scott and his men into the caves to find out what happened, while two troopers remain outside to maintain a constant scan. As they investigate, the squad is being stalked by two dark figures.
Meanwhile, in the TARDIS, Adric is arguing with the Fifth Doctor about the lack of attention and respect he receives compared to Tegan or Nyssa. Finally, he declares that he wishes to return to his home planet of Terradon, which the Doctor loudly proclaims is not possible. However, Adric is insistent that he can calculate the coordinates to guide the TARDIS back into E-Space.
Scott's squad splits into two groups, but a strange jangling sound comes over the communications system, and a flare appears on the outside scanner indicating members of the squad who are killed. Outside, Walters and Synder see more lifeforms appear on the scanner — the TARDIS crew, which have just landed in the caves. However, Scott instantly assumes that the newcomers must be responsible for the deaths, especially when he is informed that one of the new lifesigns shows two heartbeats.
The Doctor, meanwhile, is examining the fossils of dinosaurs on the cave walls and musing about how he had always wanted to find out how they died. As the wounded party is making slow progress, Snyder goes in to help, but they are attacked by the dark figures. The jamming increases, and Snyder is reduced to a pile of smoking remains. As the survivors start to investigate, they continue to be picked off one by one.
Scott and the remainder of his squad come across the Doctor and his companions and capture them. Scott accuses the Doctor of the murders and while the Doctor tries to convince him otherwise, the dark figures continue their approach. Digging away at a recent rockfall, they discover the remains of the scientists as well as a metal hatch which begins to emit a trilling sound. The dark figures pick up their pace and begin to attack as everyone dives for cover.
The Doctor realises that the attackers are androids, which is why they did not show up on the scans. The androids appear to be defending the hatch and shrug off the troopers' counterattacks. Kyle recognises the sound the androids are making, and realises that they were the ones who murdered the others. As an android focuses its gaze on the Doctor, the image is transmitted elsewhere to their masters: the Cybermen. The Cyber-Leader orders that the androids destroy them all, using the words: "Destroy them! Destroy them at once!"
Adric wanders out of the TARDIS, trying to locate the Doctor because he wants to find out what signal is being received by the TARDIS. He is able to distract one of the androids long enough for Scott and his team to destroy it. The other one is subjected to intense fire at one point, also resulting in its destruction.
The Doctor has realised that the signal is being transmitted to whatever is inside the hatch. He runs back to the TARDIS and is able to jam the transmission. He and Adric return with the Doctor's tool kit, Tegan and Nyssa ushering the rest back into the TARDIS where they will be safe as the Doctor and Adric open the hatch to discover an explosive device within. The Doctor fiddles around with the bomb and inadvertently reactivates it, but together with Adric manages to disarm it before it detonates. Returning to the TARDIS, the Doctor informs the others that they are going to trace the source of the transmission. The Cybermen are puzzled as to why the bomb failed to detonate, but when they see the TARDIS, they realize the Doctor is involved and begin to review their past encounters with the Time Lord such as The Tenth Planet, The Tomb of the Cybermen and the latest, Revenge of the Cybermen.
Meanwhile, out in space, a freighter is being inspected by Earth security forces while replenishing its supplies and crew. However, the morale of the crew is low, kept going only because of the promise of their bonus for delivering the cargo on time. Ringway, one of the bridge crew, wonders if the bonus is worth it, with several murders having already taken place. Berger seems to think that it is, and suggests Ringway adopt a more cheery attitude towards the crew to maintain their morale and his.
Having arrived on board, the Doctor and Adric explore the freighter, and come across the bodies of two dead security men. They are about to leave when Ringway comes out of nowhere, and says, "On this ship, we execute murderers!"
Ringway escorts the two travellers to meet the freighter's captain, Briggs; a middle-aged woman who is only concerned about the bonus.
Scott and Kyle start to worry about the Doctor and Adric, although only Scott says anything. Scott announces that he wants to go and find the Doctor, but Nyssa replies that the Doctor will not thank them for their efforts. Scott takes no notice and Tegan, also willing to help, goes with Scott and his remaining soldiers.
The Cyber-Leader decides that it is time to take control of the freighter, and orders a unit of Cybermen to be activated. The unit marches on the bridge, and is spotted on one of the monitors. Briggs orders a blockade set up outside the bridge, despite the Doctor pleading that her men won't stand a chance if they do that. Sure enough, the Cybermen effortlessly blast through the barricade. At the same time, Ringway turns on the crew and announces that he now works for the Cybermen, having grown tired of Briggs. The Doctor manages to disarm him, and closes the bridge doors before the Cybermen can get inside. Briggs seems confident that they can hold out until they get back to Earth, as there are only a few Cybermen, until Adric inquires as to how many cargo containers there are, when she realizes that all 15,000 are probably full of Cybermen.
As Scott, Tegan and the others kill a Cyberman and damage a second, the Cyber-Leader has his troops soften one of the bridge doors with a thermal lance. The Doctor taps into the antimatter storage system and uses it to stabilise the door just as a Cyberman breaks through, with the result that the Cyberman is fused into the door. The Cyber-Leader had also fitted explosive charges to the other bridge door, however. Just before it goes to detonate them, the Cyberman that Tegan damaged crawls up. The lieutenant points out that all the crew have been accounted for, and the Cyber-Leader erroneously concludes that Ringway lied about the crew numbers.
The explosives are detonated, destroying the bridge door. The Cybermen take the bridge, and the Leader has Ringway killed for his "deception". Additional Cybermen are revived, in order to deal with the other humans, and as they march through the hold the Cyber-Leader declares, "My army awakes, Doctor!"
The Cybermen fit a device to the ship's computer which locks it on course to collide with Earth. It then explains its mission - several galactic powers are going to meet on Earth that day and hold a conference that will unite them in a war against the Cybermen. Their original plan was to use the bomb to virtually destroy Earth, commandeer the freighter and use the army to kill anyone who survived. Due to the Doctor's interference, the Cyber-Leader has switched to its backup plan - crash the freighter into Earth. As it is powered by antimatter, the resulting blast will be every bit as devastating as the bomb would have been.
Tegan gets separated from Scott and his men, and is eventually captured. Scott returns to the TARDIS, but is followed by a pair of Cybermen. The troopers kill the Cybermen and take their guns, but Kyle is killed in the process. Tegan, meanwhile, is taken to the bridge. Noting the Doctor's reaction when she is brought into the bridge, the Cybermen comments that emotions must be a severe handicap for the Doctor. The Doctor argues that emotions are what makes life worth living - in response, the Cyber-Leader orders that Tegan be killed. The Doctor throws himself in front of Tegan, and the Cyber-Leader remarks that emotions are a disadvantage, as now he only has to threaten to kill Tegan for the Doctor to obey him.
Scott and the troopers leave the TARDIS again, as the Cyber-Leader decides to leave the ship in the TARDIS. Briggs, Berger and Adric are left on-board so that a few remaining Cybermen can observe their reactions, and thus better understand human weaknesses. The Doctor and Cybermen leave in the TARDIS, and Scott takes out the Cybermen guarding the bridge. Briggs wants to abandon ship, but Adric tells her that he might be able to override the device controlling the ship's computer. He overrides one segment of it and Berger tries to bring the ship out of warp, but this has an unexpected effect - the freighter jumps into a time warp and starts going backwards in time. Tegan is relieved at this, believing the Earth to be safe. Her relief is soon shattered when the Doctor points out that it has made the situation far worse - not only will Earth still be destroyed, but with the freighter now back in an earlier point of its history, the planet's whole existence will be nullified.
Adric overrides another part of the control device, bringing the freighter out of warp - but still perilously close to Earth. Briggs decides that it's time to abandon ship, but Adric is still determined to break the codes. Scott drags him into the escape pod, only for Adric to jump out at the last second and continue his efforts. Back on the TARDIS, the Doctor points out that they have travelled back 65 million years - at about the time the Earth collided with an object which killed the dinosaurs. After some prompting from Nyssa, Tegan realises that the object was really the freighter, and that rather than erasing Earth's history, the explosion will form a key part of it.
While Adric continues in his efforts, Scott signals the TARDIS and informs them that Adric's still on-board. The Cyber-Leader decides to kill the TARDIS crew, but Tegan jumps on him and the Doctor breaks up Adric's gold-plated Mathematics Achievement badge and feeds the fragments into the Leader's chest unit. The maddened and asphyxiating Leader fires its gun aimlessly - damaging the TARDIS console in the process - and the Doctor grabs the gun and shoots the Leader, killing it.
Adric is closing in on fully deactivating the control device, but is thwarted when a damaged Cyberman tries to shoot him. The shot misses Adric and destroys the freighter's controls instead, leaving no chance of altering the freighter's course. The Doctor tries to save Adric, while Nyssa quickly deals with the Cyber-Lieutenant, but he is unable to work the damaged TARDIS console, and he, Nyssa and Tegan watch helplessly as the freighter smashes into Earth with a massive explosion.
Then the credits start to roll in silence, with a close up of Adric's badge, broken up on the floor.
Doctor Peter Davison (Fifth Doctor)
Companions Matthew Waterhouse (Adric)
Sarah Sutton (Nyssa)
Janet Fielding (Tegan Jovanka)
James Warwick — Scott
Clare Clifford — Kyle
Steve Morley — Walters
Suzi Arden — Snyder
Ann Holloway — Mitchell
Anne Clements, Mark Straker — Troopers
Beryl Reid — Briggs
June Bland — Berger
Alec Sabin — Ringway
Mark Fletcher, Christopher Whittingham — Crewmembers
David Banks — Cyber Leader
Mark Hardy — Cyber Lieutenant
Writer Eric Saward
Director Peter Grimwade
Script editor Antony Root
Eric Saward (uncredited)
Producer John Nathan-Turner
Executive producer(s) None
Production code 6B
Series Season 19
Length 4 episodes, 25 minutes each
Originally broadcast March 8–March 16, 1982