The waters surrounding an island become contaminated by chemical dumping, and people who eat fish caught in those waters become deformed and violent.

The TARDIS arrives on the planet Telos where an Earth archaeological expedition, led by Professor Parry, is attempting to uncover the lost tombs of the Cybermen. With a lot of help from the Doctor the archaeologists enter the tombs. There, one of the party, Klieg, reveals himself and his business partner, Kaftan, to be planning to revive the Cybermen. He wants to use their strength, allied with the intelligence of his own Brotherhood of Logicians, to create an invincible force for conquest. It transpires however that the tomb is actually a giant trap designed to lure humans suitable for conversion into further Cybermen - a fate that almost befalls Kaftan's assistant Toberman. After fending off an attack by Cybermats - small but dangerous cybernetic creatures - the Doctor eventually defeats the revived Cybermen, led by their Controller, and reseals the tombs. The Controller is apparently destroyed in the process.

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, starting 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.