The TARDIS arrives in December 1986 at a South Pole Space Tracking station where the personnel, under the command of General Cutler, are engaged in trying to talk down a manned space capsule that has got into difficulty.

The Doctor realises that the problem stems from the gravitational pull of another planet that has entered the solar system and is now heading for Earth. His words are borne out when the base is invaded by a force of alien Cybermen. The Cybermen's world, Mondas, is draining energy from Earth - once its 'twin planet' - and the situation will soon become critical.

The Cybermen propose to take the humans back to Mondas for conversion into further members of their race. The humans fight back - although sabotage by Ben prevents Cutler from taking the highly dangerous step of launching a powerful Z-bomb - and Mondas eventually disintegrates due to absorbing too much energy.

All the remaining Cybermen collapse and die, having been totally dependent on their planet. The Doctor has become weak during the ordeal, and hurries back to the TARDIS...