The Tenth Planet

The Tenth Planet was the second serial of season 4 of Doctor Who. It was the final chronological appearance of William Hartnell as the First Doctor and introduced Patrick Troughton as the Second Doctor in the final moments of part four.

Due to the fact that The Smugglers was the final story in season 3's production block, however, The Tenth Planet was actually the first story produced in this season; from a production standpoint, this was William Hartnell's only contribution to season 4. It also introduced the Cybermen, who would become a longstanding enemy of the Doctor.

The Tenth Planet was a major historical turning point for the Doctor Who series by employing a new concept that would come to be known as regeneration. This plot mechanic has contributed to its long run on television. Likewise, it introduced the idea the Doctor has multiple incarnations and can regenerate to coincide with the departure of an actor playing an incarnation to avoid cold transition from one actor to the next.

Hartnell returned to reprise the role of the First Doctor one final time in 1972-73's The Three Doctors for the opening of the tenth anniversary season of Doctor Who.

As of 2019, the fourth and final episode of this serial remains missing from the BBC film archive, with only the regeneration sequence and various other short clips existing intact. However, the missing episode has been carefully reconstructed in an animated format for the DVD release in 2013 and the 2017 story Twice Upon a Time recreated parts of the missing episode for its opening sequence.

Synopsis
Many years ago, Earth's twin planet, Mondas, drifted away to the edge of space. Its inhabitants grew weak, so their scientists created spare parts for their bodies. Limbs and organs were slowly replaced by metal and plastic. Emotions were removed. The Cybermen were born.

The Doctor's TARDIS lands at the Snowcap space tracking station in Antarctica in December 1986. A routine space mission starts going wrong. When the base personnel's suspicions are roused, the Doctor informs them that the space capsule is being affected by the gravitational pull of another planet — a tenth planet in the Solar system.

The loss of a routine space mission and the appearance of that planet in the sky herald the arrival of the Cybermen, who are intent on the destruction of the Earth and the conversion of all humans into Cybermen. Ben and Polly fight to save the world, but it is a battle that may be the Doctor's very last.