The TARDIS lands near an international tracking station in Antarctica. The year is 1986, and the Doctor, Polly and Ben are just in time to witness the arrival in the solar system of Mondas, a planet which is the mirror image of Earth. Soon, Mondas begins to drain Earth of its energy, while the tracking station is invaded by the other world's natives: the Cybermen, humans who have replaced living tissue with cybernetic attachments. Polly and Ben must save their planet and stop the Cybermen from converting all humanity into creatures like themselves... but something is very wrong with the Doctor.

In early 1966, Dr Kit Pedler began acting as Doctor Who's “scientific adviser”, offering creative story ideas grounded in real-world science. His initial meeting with story editor Gerry Davis led to the development of The War Machines, the last story of Season Three. Pedler then proposed an adventure involving a vampiric life form, which was not taken forward, before suggesting a storyline about a space capsule which was drained of energy, the culprits being “star monks”. Although Davis liked the general concept -- especially given the significant public interest in the space race then under way between the United States and the Soviet Union -- he was wary of comparisons between the “star monks” and the Meddling Monk, who had last appeared in The Daleks' Master Plan in January.

Davis encouraged Pedler to think in terms of his own experiences to inspire a different sort of antagonist. A medical researcher by trade, Pedler was keenly concerned about the possibility of replacing human limbs and organs with machines which, in 1948, became known as cybernetics. Through discussions with his wife, Pedler envisaged a race of creatures which had taken this practice to its logical extreme, becoming more machine than man; he christened them Cybermen. Davis suggested that these beings could come from Earth's heretofore unknown twin planet.

The polar setting was inspired by the science-fiction classic The Thing From Another World

Pedler was commissioned to write The Tenth Planet on May 17th; the title was devised by his wife. The polar setting was inspired by the 1951 science-fiction classic The Thing From Another World, although Pedler's serial would be set in Antarctica, as opposed to the Arctic milieu of the movie. The Tenth Planet would be the first story made as part of Doctor Who's fourth recording block, but it would be the new season's second adventure, as The Smugglers would be held over from the end of the previous block.

During June, soon after starting work on The Tenth Planet, Pedler became seriously ill with ulcerative colitis, which required surgery and an extended stay in hospital. Having already contributed to the development of the storyline, Davis agreed to assist in the scripting duties as well; permission for his staff contribution was granted on June 29th. Davis would be credited as the co-author of the final two episodes, and copyright on the Cybermen would be split equally between the writers. As it turned out, Davis basically wrote the first draft of each script, concentrating on the action elements. Pedler then wrote a second draft, filling in more scientific details. The pair cooperatively completed a third and final draft.

Meanwhile, producer Innes Lloyd had obtained permission from new Head of Serials Shaun Sutton to replace William Hartnell as the Doctor. Hartnell's health was deteriorating, and his presence during production was becoming increasingly disruptive. Although he was heartbroken by the prospect, Hartnell consented to leave Doctor Who after making one further serial; this would be The Tenth Planet. To explain the change of lead actor, Davis posited that, since the Doctor was an alien, he could die and come back to life in a new body. Lloyd expanded on this idea, suggesting that this “renewal” could be a recurring ability of the Doctor's, via which he could transform himself from an elderly man to a younger one. The Tenth Planet was amended to incorporate the change of lead actor at its conclusion; originally, the Doctor, Polly and Ben simply left in the TARDIS, to the bewilderment of Barclay and Dyson.

In late June, Lloyd and Davis approached well-known character actor Patrick Troughton about taking over the role of the Doctor. Although he was interested in the part, Troughton was wary of committing himself to such a demanding and highly-visible role, let alone to a programme which had already been on the air for three seasons. The actor therefore deferred his decision to later in the summer. By the end of July, however, the opportunity to earn a regular paycheque had become too tempting for Troughton to resist. He met with Lloyd, Sutton, and Head of Drama Sydney Newman and agreed to become the Second Doctor. On August 2nd -- the same day that Hartnell's imminent departure was announced to the press -- Troughton signed a contract to appear in twenty-two episodes as the new Doctor, comprising five serials.

In the scripts, the Cybermen had human faces with a metal plate under their hair

The director assigned to The Tenth Planet was Derek Martinus, who had handled Galaxy 4 and Mission To The Unknown a year earlier. Responsibility for realising the Cybermen fell to costume designer Sandra Reid, who was working on Doctor Who for the first time. In the scripts, the Cybermen had human faces -- although each was similar to the others -- with a metal plate under their hair. Their hands were also human, although their arms were transparent, made up of rods and lights. After consulting Pedler, Reid simplified these ideas, dressing the Cybermen in jersey material which covered their faces, and surmounting the body with polythene. A large metal headpiece incorporated a truck headlamp. The scripts also suggested the inclusion of a movable arm which could extend from each Cyberman's chest unit, but such an accessory was too costly to implement.

The BBC Television Film Studios in Ealing, London played host to model filming for The Tenth Planet on August 30th, followed by regular pre-filming from August 31st to September 2nd for material set on the Antarctic plain. The Cyberman costumes made their first appearance at Ealing, and were so hot under the studio lights that some of the actors fainted. Michael Craze also found this work difficult. He had just undergone surgery to remove a bone chip from his nose, and the polystyrene “snow” thrown into a wind machine by production assistant Edwina Verner caused severe nasal irritation. As it happened, though, Craze wasn't deterred from asking Verner out on a date; they would subsequently marry. Meanwhile, on September 1st, the BBC confirmed that Troughton had been cast as the new Doctor.

Work then shifted to videotape at Doctor Who's regular production home of Riverside Studio 1 in Hammersmith, London. Whereas studio recording for Doctor Who had occurred on Fridays during its first three recording blocks, the new set of episodes instead shifted to Saturdays. This meant that rehearsals could begin on Tuesdays, and members of the regular cast would no longer have to miss parts of the first day in order to participate in pre-filming for the next serial in production, which took place on Mondays. As such, the first episode of The Tenth Planet was taped on Saturday, September 17th. As with The War Machines, Bernard Lodge designed a special animation sequence for The Tenth Planet to announce the story title, episode number and author. Unfortunately, the graphics for the first installment incorrectly listed the writer as “Kitt” Pedler.

Following the completion of Episode Two on September 24th, William Hartnell became ill with bronchitis. Fortunately, the script for Episode Three already sidelined the ailing Doctor from the main action; Davis now amended it to have the Doctor collapse in the very first scene. The Doctor's dialogue was shared between Ben, who would now relate things the Doctor had told him, and Barclay, who was assigned the more technical dialogue. Hartnell was given the week off and Gordon Craig, who had doubled for him in the Ealing material, played the unconscious Doctor during recording on October 1st. Lodge's animation was again marred by a typographical error, misspelling Davis' surname as “Davies”.

October 8th marked the end of William Hartnell's tenure on Doctor Who

October 8th marked the end of Hartnell's tenure on Doctor Who. The first scene recorded on this day was the renewal sequence itself. Originally, this would have simply seen Hartnell collapse on the floor of the TARDIS console room with his cloak covering his face; in the first episode of the next story, The Power Of The Daleks, the cloak would have been pulled back to reveal Troughton instead. However, vision mixer Shirley Coward was able to design a way to depict the renewal on-screen. This took advantage of a malfunctioning mixing desk in Studio 1, which caused the image to flare noticeably. Thus Coward created an effect in which Hartnell's image became overexposed almost to a white-out, then settled down again to reveal Troughton's countenance. Accordingly, Troughton was asked to sign a new contract on September 16th to appear in the scene. Once this was completed, the remainder of Episode Four was taped, whereupon cast and crew retired to a farewell party in Hartnell's honour at Lloyd's home.

The concluding installment of The Tenth Planet was broadcast on October 29th. As of 6.15pm that evening, the William Hartnell era of Doctor Who was over, after thirty-five months, twenty-nine stories and one hundred and thirty-four episodes. It would remain to be seen whether the British public would accept anybody else as the Doctor...

• Doctor Who Magazine #214, 6th July 1994, “Archive: The Tenth Planet” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #7, 12th May 2004, “Bye Bye Blues” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #8, 2016, “Story 29: The Tenth Planet”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The First Doctor by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.