The TARDIS lands at the South Pole in December 1986 at a Space Tracking Centre. The personnel, under the command of General Cutler, are trying to rescue a manned space shuttle that has gotten into difficulty.
The Doctor realises that the problems stem from the gravitational pull of a tenth planet that has entered the solar system. Soon the occupants of the planet arrive on Earth - the Cybermen.
The planet, Mondas, a twin planet of Earth, is draining energy from Earth. The Cybermen propose to take humans back to Mondas and turn them into Cybermen.
The humans fight back, and fortunately Ben sabotages the Z-bomb which would cause mass devastation. The Cybermen, who are dependant on their planet, collapse and die when Mondas explodes after absorbing too much energy. The Doctor, weakened by the events, retires to the TARDIS, and begins his regeneration ...
