Written by Derrick Sherwin
Story by Kit Pedler
Directed by Douglas Camfield
Narrowly evading a missile fired at them from a spaceship in Earth's orbit, the TARDIS materializes in England of the near future. Discovering that the TARDIS is damaged, the travellers attempt to reach their old friend Prof. Travers for help. However, upon reaching Travers' home they encounter Isobel Watkins, who lets them know that Travers is away and she and her uncle Professor Watkins, are living there temporarily. Isobel is concerned as her uncle has recently gone missing. The Doctor offers to help track him down, starting at this work-place, the London HQ of International Electromatics, the world's major supplier of electronic equipment.
Meeting IE's managing director Tobias Vaughn, the Doctor is immediately suspicious. Soon after, the Doctor and his friends encounter old ally Colonel Lethbridge-Stewart, now promoted to Brigadier and in charge of the British branch of the United Nations Intelligence Taskforce (UNIT). UNIT is investigating a series of odd disappearances that seem to be connected to International Electromatics. The Brigadier asks the Doctor to help him in his investigation.
The Doctor discovers that Vaughn is working with the Cybermen in a plan to invade Earth, though Vaughn's try aim is to betray them and seize control for himself. Professor Watkins is being forced by Vaughn to develop a machine that will generate emotional impulses in the Cybermen, a process that causes the cybernetic beings to die or go insane.
Using a hypnotic signal broadcast through IE equipment and products, the Cybermen immobilize most of Earth's population. However, the Doctor has managed to protect himself and UNIT from the signal and they mount a counterattack, with the reluctant aid of Vaughn himself. Together, they successfully fight off the invasion.