The Tenth Planet, Episode Four

The TARDIS materialises decades in the future, in the year 1986, at Snowcap Tracking Station at the South Pole. There, an international team is struggling to bring down a shuttle which has inexplicably been drawn out of orbit - by the gravity of a tenth planet, Earth's exact mirror twin. The Doctor realises what's about to happen, but before he can deliver his warning, Snowcap is invaded by the Cybermen - natives of the planet Mondas who have slowly replaced most of their body parts with cybernetic prostheses, and who have now returned to their twin to drain the energy away from Earth. However, the Doctor and his friends soon find themselves fighting a threat closer to home - in order to save his astronaut son, the American General Cutler intends to attack Mondas with a Z-Bomb despite the danger that the radiation from the explosion will render half of Earth uninhabitable. The Doctor realises that the Cybermen have invaded Earth because Mondas is drawing away too much energy, and that their best hope is simply to stall the Cybermen until Mondas explodes of its own accord. But the energy drain is affecting the elderly Doctor's health, and when he finally returns to his TARDIS he collapses - and before Ben's and Polly's eyes, he seems to change into an entirely different person...