Clara admonishes the Doctor for his treatment of Courtney Woods, so he decides to take both teacher and pupil on a trip to the Moon in the year 2049. But there is a mystery afoot on the lunar surface: the gravity is wrong, the landscape is wracked by seismic activity, and spider-like monsters lurk in the shadows. With tidal chaos wreaking havoc on Earth, Captain Lundvik arrives on a recycled space shuttle, determined to solve the crisis -- or destroy the Moon with an arsenal of nuclear weapons. When the Doctor refuses to become involved, Clara is faced with a terrible decision which may affect the future of the human race.

Peter Harness was a lifelong Doctor Who fan who inquired about potentially writing for the series shortly after the 2003 announcement that it would be returning to television. At this point, however, Harness' career was still in its nascent stages, and nothing came of his approach. By 2011, Harness' resume had grown extensively, prompting the Doctor Who production office to contact him about contributing to Season Thirty-Three. Executive producer Steven Moffat was looking for scripts which would form the second half of the run, to air in the spring of 2013. With companions Amy Pond and Rory Williams set to leave during the season's early episodes -- which would be broadcast in the autumn of 2012 -- Moffat planned to pair Matt Smith's Eleventh Doctor with a Victorian governess called Beryl. She would divide her time between adventuring in the TARDIS and caring for two children named Charlotte and Godfrey.

Harness' initial proposal for Doctor Who was called “When We Weren't There”, and was deemed unsuitable by the production team. However, he had already conceived a second story idea, in which the Moon would be revealed to be an enormous alien egg. This audacious notion was felt to have more potential, and Harness was asked to develop it further. Drawing upon Beryl's background, he suggested that one of her charges should be involved in the proceedings, accompanying their governess when the Doctor took them to the Moon.

It would not be until 2013 that Peter Harness was in a position to resume work on the Moon narrative

Soon afterwards, however, the BBC offered Harness a major telefantasy project: a prestigious adaptation of Susanna Clarke's best-selling alternative-history novel Jonathan Strange & Mr Norrell. As a result, Harness set his Doctor Who script aside, and it was removed from consideration for Season Thirty-Three. It would not be until 2013 that Harness was in a position to resume work on the Moon narrative for inclusion in Season Thirty-Four, by which time Doctor Who had undergone a number of changes. Not only had the Beryl character been replaced by the modern-day Clara Oswald, but Smith was in the process of leaving the show, and so Harness' adventure would instead feature Peter Capaldi's Twelfth Doctor.

Far from being to the detriment of the story, Moffat felt that these alterations played to its strengths. He wanted the midpoint of Season Thirty-Four to depict a dramatic rift in the relationship between the Doctor and Clara. This development would come on the heels of -- and be exacerbated by -- the icy rapport which would develop between the Time Lord and Clara's boyfriend, Danny Pink, in episode six, The Caretaker. As such, Harness' narrative was positioned as the year's seventh installment. It was given the intentionally lurid title Kill The Moon, which echoed another pivotal mid-run adventure, Let's Kill Hitler, from Season Thirty-Two.

Although Beryl's charges had evolved into Angie and Artie Maitland, to whom Clara served as a temporary nanny during Season Thirty-Three, they had been dropped from Doctor Who following the decision to establish Clara as a schoolteacher. Harness still wanted a young character to travel to the Moon, and he suggested that the Doctor and Clara could be joined by a Coal Hill student whom he dubbed Emma. As he began to work on his storyline around September, however, Harness was given the chance to read some of the existing draft scripts for Season Thirty-Four. Amongst them was The Caretaker, for which writer Gareth Roberts had developed a disruptive student called Courtney Woods. Courtney had already found favour with the production team, who had decided to incorporate brief appearances by the character in the season's first two episodes. It was now agreed that Courtney would take over Emma's role in Kill The Moon.

Captain Blinovitch was revealed to be Courtney's future self

Harness' first draft was completed towards the end of January 2014. At this stage, Kill The Moon opened with a prologue which depicted the deaths of the Mexican astronauts, and the decision to send the last serviceable space shuttle to investigate. Henry and Duke were the world's last living astronauts, and they were to be accompanied by Captain Blinovitch. When the TARDIS appeared, it was immediately evident that Blinovitch knew all three of its occupants, although she remembered Clara as “Mrs Pink”. It was Courtney, rather than Clara, who made the decision to ignore humanity's wishes and save the creature hatching from the Moon. Blinovitch was then revealed to be Courtney's future self; the presence of the same person at two stages of her life had allowed her to change history. This was described as the Blinovitch Limitation Effect, which had been referenced in Doctor Who as far back as 1972's Day Of The Daleks.

Around the end of February, it was decided that the Blinovitch subplot was taking the focus away from the deterioration of the relationship between the Doctor and Clara. This entire strand, including any connection between Courtney and Blinovitch, was abandoned. Indeed, Blinovitch was eventually renamed Lundvik, with the surname retained only in the Doctor's account of Courtney's future history. Furthermore, Henry was no longer a former astronaut, but a retired member of the Virgin Galactic cabin crew. This reference to the spaceflight company founded by billionaire Richard Branson would be deleted in editing. Also excised was the use of an old Moon buggy, which would have been hotwired and used for transportation around the lunar surface. One allusion to Doctor Who's past which survived the drafting process was the Doctor's admonition against “hanky panky” in the TARDIS. This dialogue nodded to the phrase used by former producer John Nathan-Turner to rule out any suggestion of a sexual attraction between the Doctor and his companions.

Kill The Moon was paired with the season's eighth episode, Mummy On The Orient Express, as Block Four of the production calendar. The director was Paul Wilmshurst, who had been in discussion to work on Doctor Who as far back as 2010, but had been precluded from doing so until now due to scheduling conflicts. After reading Harness' script, Wilmshurst's principal concern was how sequences on the surface of the Moon would be realised. Green screen effects in the studio were one option, as was the use of a quarry. However, the director's preference was to film abroad in Lanzarote, the easternmost of Spain's Canary Islands off the coast of Morocco. Wilmshurst had holidayed there in 1986, and he had been particularly struck by the volcanic soil which was responsible for the cratered landscape of the Parque Nacional de Timanfaya. Indeed, the same area had been used for astronaut training by both the National Aeronautics and Space Administration and the European Space Agency.

Steven Moffat and Peter Harness agreed that the script would be jokingly renamed “Return To Sarn”

During March, Wilmshurst undertook a reconnaissance trip to Lanzarote, after which the production team concurred that this was the best option for the Moon sequences. Moffat observed that Doctor Who had filmed on the island before: during the making of 1984's Planet Of Fire, Lanzarote had represented both itself and the alien world of Sarn. As such, he and Harness agreed that the script would be jokingly renamed “Return To Sarn”, and this title remained in use through to the start of production.

The first week of filming for Kill The Moon took place at Roath Lock Studios in Cardiff. May 5th saw recording on the set for the shuttle storage bay, including the Doctor testing the gravity with his yo-yo. At Capaldi's request, the prop was designed to resemble the toy used by the Fourth Doctor, Tom Baker, in 1975's The Ark In Space. The actor's anxiety about his limited skill with a yo-yo was assuaged when his daughter Cecily's boyfriend offered to spend a morning tutoring him. Scenes in the storage bay were completed on the 6th, after which Wilmshurst moved over to the moonbase set; it would remain his focus through to the 9th, when production stood down for the weekend.

On May 12th, essential cast and crew flew to the Canary Islands, although only plate shots were on the itinerary for the first day on Lanzarote. Unlike Planet Of Fire, which had made use of several different venues on the island, Wilmshurst had decided to confine his shots for Kill The Moon to a single location. This was the Parque Natural de los Volcanes, immediately to the east of the Parque Nacional de Timanfaya, near the Caldera de los Cuervos. Work there continued through to May 15th; the footage would be graded in post-production and enhanced with computer-generated imagery to replicate the whites and greys of the Moon.

Wilmshurst's team returned to the United Kingdom on Friday the 16th, and then enjoyed a weekend off before filming resumed at Roath Lock on May 19th. Further moonbase sequences were completed on this day, followed by action on the standing TARDIS set. Recording on the 20th began at the Power House in the Cardiff Bay Business Centre, to which the set for Clara's apartment had been relocated. Material in Coal Hill School was then taped at the former St Illtyd's Boys' College in Cardiff. Green screen shots of a hologrammatic Doctor were also taped there, for an ultimately-deleted scene which was to accompany Courtney's use of the DVD to control the TARDIS.

On May 21st, Aberavon Beach in Port Talbot provided the setting for the Doctor and company watching the new Moon appear in the sky. Then it was back to Roath Lock for the preceding shots of the Doctor collecting Clara, Courtney and Lundvik from the moonbase, and the intervening TARDIS journey. The remaining TARDIS scenes were completed in the studio on May 26th and June 5th; the latter day marked the end of Ellis George's time playing Courtney Woods. Part of the 5th was also taken up with effects shots of flying debris for the shuttle crash.

Kill The Moon was found to be severely over-length, resulting in a number of cuts during post-production. Originally, Clara revealed that Courtney had stolen -- and broken -- the Doctor's psychic paper in order to falsify her age. Also dropped was the shuttle crew introducing themselves, meaning that it would be well into the episode before any of the trio was named on-screen; indeed, Lundvik would not be identified until the final scene on the moonbase. Lost was much of the discussion about why space exploration had ceased, which formed a commentary on the modern obsession with smartphones and tablet computers. This notion survived in the Doctor's explanation that humanity would one day venture out into the galaxy because “something occurred that made it look up, not down.”

• The Doctor Who Companion -- The Twelfth Doctor: Volume Two, October 2019, “Kill The Moon” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #78, 2017, “Story 248: Kill The Moon”, edited by Mark Wright, Hachette Partworks Ltd.