The TARDIS materializes near the Moon. A missile attack forces the time-travellers to land in England. The visual stabiliser is damaged, and the TARDIS is invisible. They hitch a ride to London to ask Professor Travers (whom they worked with in Tibet and London) for help. The taciturn driver who picks them up believes they are trying to escape danger, as they were inside the compound of the International Electromatics Company, a vast and powerful network of industrial complexes. There are armed guards at a checkpoint, and there is an air of cloak and dagger about the whole business. In fact, after letting off his riders, the driver is confronted by armed men and when he refuses to come with them because they have no authority outside the complex, they shoot him.
In London, the Doctor and companions find that Professor Travers is in America. A certain Professor Watkins (Edward Burnham) has taken over his flat but has gone missing after working with International Electromatics. The Doctor and Jamie check out the company’s office tower and, after getting the run-around from an officious computer receptionist, are taken to see the Managing Director Tobias Vaughan (Kevin Stoney). Vaughan’s sadistic head of security wants to “interrogate” them, but Vaughan says no. After they leave, he contacts the Cyber-Planner.
The Doctor and Jamie are followed by mysterious figures in a car, which turn out to be from UNIT (United Nations Intelligence Taskforce). They are picked up by the military and taken to see Brigadier Lethbridge-Stewart (Nicholas Courtney), who is watching International Electromatics like a hawk. It seems the missing lorry driver was a UNIT agent. Bored at the flat, Zoe and Professor Watkins daughter Isobel (Sally Faulkner) leave to investigate the company on their own. Zoe attracts a great deal of attention; offended by the computer receptionist, she poses an insoluble problem and it blows up. They are captured, and then the Doctor and Jamie are captured when they come looking for them.
In a sprawling countryside complex, they learn about the Cerebration Device, they find Professor Watkins being forced to work on a secret project, and they discover a living creature in a cocoon. Vaughan knows the Doctor has a machine that travels to other planets. He realizes that his mysterious allies fear the Doctor. UNIT rescues the Doctor and his friends spectacularly, with a helicopter, breaking UNIT’s cover. Vaughan is apparently working with alien invaders. Out of one of the cocoons comes a Cyberman. More Cybermen are infesting the London sewers, preparing to take over the Earth. The Doctor and Jamie see one hatching from its cocoon. The Cybermen are controlling the minds of bureaucrats in the British government, but UNIT answers to the World Authority, not to them.
Vaughan is planning to betray the Cybermen after they conquer the Earth for him. He revives one from its cocoon, shoots it with Professor Watkin’s machine and makes it mad with fear. It blunders off into the sewers, shrieking in terror. Isobel and Zoe head into the sewers to photograph the Cybermen as proof that they exist, angry that the Brigadier wants his own all-male photographic unit to do the job. Jamie trails after them. They get a Bobbie killed, and some UNIT troops. The Cyberman, mad with fear, stumbles past them, and UNIT takes him out with grenades. In a rather chilling scene, Cybermen emerge from manholes and march through the streets of London. Vaughan turns the machine on his own minions and threatens the Professor, but UNIT manages to rescue the Professor to help the Doctor.
The Doctor makes his way through the sewers into Vaughan’s headquarters, hoping to convince him that the Cybermen will betray him. In his arrogance, he is certain he can control them. Zoe uses her math skills to program UNIT’s missiles to cause a chain reaction among the invading Cyberman ships. The Cybermen blame Vaughan for this and order the annihilation of all life on Earth using their Cyber-Megatron bomb, which will leave Vaughan the ruler of a dead planet. The megalomaniac agrees to help the Doctor, not because he gives a damn about humanity, but because the Cybermen have betrayed him, as he was going to betray them.
Kevin Stoney was quite watchable as an increasingly arrogant sociopath in the throes of full-on eye-twitching insanity. The scene of climbing up a rope-ladder into the helicopter was problematic. Nobody minded Sally Faulkner’s skirt being blown up around her waist, but Jamie’s Kilt was the centre of a publicity campaign about what was under it, so he had lead weights sewn into the hem like Queen Elizabeth does. The first and fourth episodes of the eight-part serial were lost and reconstructed with animation, the first story ever treated in this way. It was praised as an exciting suspense and action tale, and Zoe’s feisty characterization was particularly appreciated.