Final adventure for the first incarnation of television's favourite time traveller. The Doctor (William Hartnell), Ben and Polly arrive at the South Pole Tracking station in 1986, just as a strange satellite enters Earth's orbit, affecting the latest space mission. The Doctor predicts that the new arrival is Mondas, Earth's long-lost twin planet, and is proved correct when the base is invaded by the Cybermen, the planet's ruthlessly logical and seemingly invulnerable inhabitants.