The Tenth Planet is the partly missing second serial of the fourth season in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from 8 to 29 October 1966. It was William Hartnell's last regular appearance as the First Doctor, and the first story to feature the process later termed regeneration, whereby the lead character, The Doctor, undergoes a transformation into a new physical form. Patrick Troughton makes his first, uncredited appearance as the Second Doctor. The serial is also notable as the first story to feature the Cybermen, a race of malevolent cyborgs that became a recurring adversary in later Doctor Who stories. The "tenth planet" in the title makes reference to a fictional lost planet in Earth's Solar System; at the time of production, the Solar System was generally held to consist of nine planets, prior to the redesignation of Pluto as a minor planet. The Tenth Planet is an incomplete Doctor Who serial – one of many serials that were affected by the BBC's policy of wiping archived programmes in the 1960s and 1970s. Only three of the four episodes are currently held in the BBC archives; the last episode remains missing, although several short clips, including the regeneration sequence, have been discovered intact. In 2013, The Tenth Planet was released on DVD with a full-length animated reconstruction of its missing footage. Plot The Doctor and his companions Ben and Polly arrive in the TARDIS at the South Pole in the year 1986. They find themselves in the Snowcap Base, a space tracking station commanded by General Cutler. The base is supervising the mission of the Zeus IV spaceship, running a routine probe on the Earth's atmosphere. The spaceship is drawn off-course by an unknown force, and Snowcap monitoring staff discover a new, unknown planet approaching Earth. Recognising identical landmasses to those of Earth, the Doctor reveals that this is Mondas, the Earth's long-lost twin planet, and that its inhabitants will soon be visiting Earth. A mysterious spaceship lands in the snow and three robotic creatures emerge, kill the guards and infiltrate Snowcap Base, taking control. They reveal that they are Cybermen, a race who, though once like human beings, have gradually replaced their bodies with mechanical parts, and eliminated the "weakness" of emotion from their brains. The Cybermen prevent the base staff from saving the Zeus IV, and it