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 1986, and are taken to 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 is destroyed by the gravitational pull of Mondas. The emotionless Cybermen state that the lives of the crew are irrelevant to them. The Cybermen explain that Mondas is absorbing energy from Earth and will soon destroy it. They propose to take humans back to Mondas and turn them into Cybermen.
General Cutler, the Snowcap base personnel and the Doctor's companions mount a resistance to the Cybermen, overpowering them and killing them with their own cyberweapons. Cutler plans to destroy Mondas using a Z-bomb, one of a series of powerful nuclear bombs that are placed at strategic points around the world, and contacts Space Command HQ in Geneva. The chief scientist Dr. Barclay expresses concerns that the radiation caused by the exploding planet would cause immense loss of life on Earth, and Ben argues that Mondas might destroy itself anyway when it absorbs too much energy. Suddenly, the Doctor passes out, ill. Faced with dissent, Cutler orders Ben to be imprisoned in a cabin with the Doctor. Ben escapes and, with the help of Polly and Barclay, sabotages the Z-bomb rocket. Cutler attempts to fire the Z-bomb, but the engines fail on the launchpad. As Cutler threatens to kill Ben, Barclay, and the Doctor, who has regained consciousness, he is killed by a new squadron of Cybermen. The Doctor, who informs Polly his body is “wearing a bit thin”, and realising that Mondas is approaching destruction, attempts to mediate with the Cybermen, offering them a home on Earth. The Cybermen take Polly and the Doctor back to their spaceship as hostages.
