The TARDIS materialises in England in the 1970s and the time travellers meet a girl named Isobel whose uncle, computer scientist Professor Watkins, has recently gone missing. The Doctor offers to help track him down, s...展开tarting at the place where he last worked - the London HQ of International Electromatics, the world's major supplier of electronic equipment.
His initial suspicions about IE's managing director, Tobias Vaughn, are confirmed after he becomes reacquainted with Lethbridge-Stewart, now promoted to Brigadier and in charge of the British branch of the United Nations Intelligence Taskforce (UNIT), and learns that there have been other disappearances at IE premises.
He discovers that Vaughn is in league with the Cybermen in a plan to invade Earth, but is also plotting against them with the aim of seizing power for himself. Watkins is being forced by Vaughn to develop the cerebration mentor, a machine designed to generate emotional impulses as a weapon against the Cybermen.
The Cybermen immobilise most of Earth's population - sending a hypnotic signal through special circuits incorporated in all IE equipment - and launch their invasion. The Doctor has managed to protect himself and his friends from the signal and - with help from an embittered Vaughn, who is killed during the fighting - the invasion is eventually defeated.