The Pope summons the Doctor to the Vatican. The Haereticum, its library of banned and blasphemous texts, houses an ancient document called the Veritas -- “truth” -- written in a lost tongue. Now it has been translated, but every person who has read it has taken their own life. Seeking an explanation, the Pope asks the Doctor to read the Veritas himself. But while Bill and Nardole investigate a mysterious portal, the blind Doctor is stalked by corpse-like alien Monks. Meanwhile, in another time and place, long ago, Missy has been condemned for her crimes. The Doctor must decide whether she deserves execution... or redemption.

From an early stage, Doctor Who executive producer Steven Moffat planned to treat the initial episodes of Season Thirty-Six as a soft relaunch of the show. Intended to appeal to new and lapsed viewers, it would see the programme reintroduced through the eyes of companion Bill Potts. There would be an altered status quo -- with the Doctor now lecturing at St Luke's University, while he and Nardole guarded a mysterious vault -- and a focus on straightforward adventures which would emphasise the relationship between the Doctor and Bill. However, Moffat did not intend for this approach to persist throughout all twelve episodes: by mid-season, he wanted to shake things up by taking Doctor Who into heretofore unexplored territory.

The nature of these disruptive developments was not decided immediately. Instead, Moffat allowed time for various scripts to evolve over the first part of 2016. By the end of May, Jamie Mathieson had added a subplot to episode five, Oxygen, in which the Doctor was temporarily blinded as a result of exposure to the vacuum of space. During the summer, he suggested doing away with its quick resolution at the end of his story, to which Moffat readily agreed. He saw potential in the Doctor confronting an unprecedented limitation, and he felt that it could instigate a linked series of adventures which would also include scripts being developed by Peter Harness and Toby Whithouse -- ultimately, The Pyramid At The End Of The World and The Lie Of The Land.

Steven Moffat was struck by the sophistication of computer-controlled characters in recent video games

To bridge the gap between the end of Oxygen and the other two stories, Moffat decided to develop a narrative of his own, which would occupy the sixth slot in the season schedule. The basis for his storyline would be the simulation hypothesis, a modern take on the ancient argument that reality is illusory. The notion that human existence is actually a highly-advanced computer simulation had been the basis for various science-fiction stories, such as the 1999 blockbuster The Matrix. It had most recently received renewed academic consideration in response to the writings of Swedish philosopher Nick Bostrom. An avid player of video games, Moffat had also been struck by the increasing sophistication of the interaction offered by computer-controlled “non-player characters” in many recent titles.

As such, Moffat decided to write a story in which the Doctor discovered that he was actually a simulation, living in an artificial reality. This knowledge would provide him with the opportunity to communicate with the real Doctor, forewarning him of the plans of the Monks, the villains of Harness' and Whithouse's adventures. Inspired by Dan Brown's 2003 novel The Da Vinci Code, in which a university professor solved a series of puzzles to uncover the secrets of the early Catholic Church, Moffat chose the Vatican as his script's setting. He received permission from the BBC to include an appearance by the Pope, whom Moffat knew would have to be portrayed sensitively to avoid causing any religious offence.

The episode would also resolve the mystery of how and why the Doctor became responsible for guarding the vault, as well as the identity of its occupant. Having briefly considered using Davros, the creator of the Daleks, for this role, Moffat instead opted for Missy, the Doctor's Time Lord arch-nemesis. Michelle Gomez had last appeared as the villain in the Season Thirty-Five premiere, The Magician's Apprentice / The Witch's Familiar, and she was eager to return to Doctor Who.

Moffat was writing his script by mid-October, with a first draft completed around the start of November. The text at the heart of the story was originally given the Italian title La verità before becoming the Latin Veritas. The story itself was accorded a complementary Latin title: Extremis, in reference to the phrase in extremis, meaning “at the point of death”. The inclusion of CERN -- the European Organisation for Nuclear Research, known by the French acronym which stood for Conseil Européen pour la Recherche Nucléaire -- was inspired by Moffat's visit to the Swiss facility during the summer, when his son Joshua had an internship there.

Extremis was paired with The Pyramid At The End Of The World as Block Five of the production calendar. It was directed by Daniel Nettheim, who had made The Zygon Invasion / The Zygon Inversion for Season Thirty-Five. Recording for began with three days at Cardiff's Roath Lock Studios, from November 23rd to 25th. The first day dealt with scenes in the TARDIS and the Doctor's office. Recording on the middle day began with the clergy waiting outside the office, followed by material in the vault chamber and the White House room with the television. The Oval Office itself was the focus on the last day, prior to a break for the weekend.

Peter Capaldi was away the following week, during which he was scheduled to record his contributions to the family film Paddington 2. As such, the only work on Extremis took place on November 28th. The set for Bill's home was again part of the backlot at Roath Lock used by the Welsh-language soap opera Pobol y Cwm; Jennifer Hennessy returned to play Moira, as in the season premiere, The Pilot, and in deleted material for episode four, Knock Knock.

Filming for Extremis didn't resume until December 7th. As usual, the Doctor's lecture hall was actually the Reardon Smith Lecture Theatre, part of the Nation Museum of Wales in Cardiff. The papal convoy then pulled up outside Cardiff University that evening; this sequence was deleted in editing. The 8th and 9th saw Michelle Gomez rejoin the Doctor Who team for the first time in more than twenty months. The scenes involving her putative execution were staged at St Andrews Quarry in Dinas Powys.

It would be nearly a month before Extremis went before the cameras again, owing to both the Christmas holidays and recording for The Pyramid At The End Of The World. On January 3rd, 2017, not one but two post-secondary institutions provided venues for Moffat's script. First, Cardiff University's Glamorgan Building offered a space suitable as the Pentagon office. Then, the School of Management at Cardiff Metropolitan University posed as CERN.

Amongst the names visible on Piero's laptop was that of Rob Hull, who held the world record for the largest Dalek collection

The rest of Extremis was taped at Roath Lock, starting on January 9th with material at the conjunction of the portals. Nettheim started in the under-chapel on the 10th, before moving to the Haereticum; cast and crew would remain there through the end of the week, on January 13th. Amongst the names visible on Piero's laptop were those of Bill Pullman (who played convicted murderer Oswald Danes during the 2011 season of the spin-off series Torchwood), Philip Bond (Ganatus in 1963's The Daleks), standby art director Christina Tom, and Rob Hull, who held the world record for the largest Dalek collection. Additional Haereticum sequences were recorded on January 16th, alongside several inserts. Lastly, the news report which appeared on the White House television was taped on the 17th.

Extremis aired on May 20th at 7.25pm, five minutes later than the standard starting time for the initial episodes of Season Thirty-Six. Doctor Who was now followed by the tenth season of the quiz show Who Dares Wins, which had previously been airing earlier in the Saturday schedule.

• Doctor Who: The Complete History #87, 2018, “Story 270: Extremis”, edited by John Ainsworth, Hachette Partworks Ltd.