The Tenth Planet is a serial in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from October 8 to October 29, 1966. It was William Hartnell's last regular appearance as the First Doctor, and the first story to feature the Cybermen. Patrick Troughton also makes his first, uncredited appearance as the Second Doctor.
The episode takes place in 1986. Earth's sister planet, Mondas has returned home, bringing with it the emotionless, unstoppable Cybermen. At the Snowcap base in Antarctica, the invasion begins, but the Cybermen have not reckoned on the First Doctor and his companions. However, the Doctor becomes weaker throughout the episode, eventually collapsing and regenerating into a new form: the Second Doctor.