The TARDIS materialises in December 1986 at the entrance to a South Pole Space Tracking Station under the command of General Cutler (Robert Beatty). The scientists there are experiencing problems in controlling the return to Earth of a manned space capsule, and the Doctor realises that this is due to the gravitational pull of another planet which has entered the solar system and is now heading for Earth. His prediction is borne out when the base is invaded by a force of alien Cybermen (Gregg Palmer, Reg Whitehead, Harry Brooks, Bruce Wells, John Haines, John Slater, John Knott; voiced by Roy Skelton, Peter Hawkins).

The Cybermen's world, Mondas, is draining energy from the Earth, and the situation will soon become critical. Although Ben and Cutler manage to destroy the first wave of attackers, the base is then overrun by a second. However, one of the scientists, Barclay (David Dodimead), suddenly realises that the invaders are susceptible to radioactivity, and this suggests a means of fighting back. Using hand-held uranium rods, Ben and a group of the scientists are able to hold off and kill a number of Cybermen. At the story's climax, Mondas disintegrates due to absorbing too much energy, and all the remaining Cybermen collapse and die, having been totally dependent on their planet.

The Doctor has become steadily weaker during the course of the adventure, and after the defeat of the Cybermen he hurries back to the TARDIS. Polly and Ben follow, and find him collapsed on the floor of the control room. As they watch, his face is transformed into that of a much younger, dark-haired man (Patrick Troughton).