To test Missy's rehabilitation, the Doctor has her take his place alongside Bill and Nardole while investigating a distress signal. It emanates from an enormous colony ship, which is struggling to free itself from the gravitational pull of a black hole. To make matters worse, the vessel is overrun with lifeforms who seem to have appeared from nowhere. When Bill is badly injured, the invaders take her away to the lower decks, where time passes more quickly. There she discovers people submitting to eerie surgeries which will make them something other than human... and meets a kindly man who hides a terrible secret.

Over the course of 2016, the watershed nature of Season Thirty-Six's two-part finale gradually became apparent. It had long been earmarked as the Doctor Who swansong for executive producers Steven Moffat and Brian Minchin; indeed, Moffat had expected to exit the show with the 2015 Christmas special, The Husbands Of River Song, but he had deferred his departure due to the limited availability of his successor, Chris Chibnall. Although Chibnall had invited Peter Capaldi to remain on Doctor Who, the series star had come to feel that it was time to move on to new projects, clearing the way for Chibnall to cast a Doctor of his own. Pearl Mackie and Matt Lucas would be winding up their time as companions Bill Potts and Nardole, since Moffat wanted to avoid saddling Chibnall with established characters. And Michelle Gomez had decided that she should likewise give up the role of Missy, whom she felt was ideally matched opposite Capaldi's Doctor.

Consequently, Moffat approached the finale knowing that it would have to write out virtually all of his regular and recurring characters. He had already prepared for Bill's departure by introducing her love interest, Heather, who acquired substantial alien powers in the season premiere, The Pilot. Moffat knew that he could therefore place Bill in terrible circumstances as a final gauntlet to test the character. Capaldi had long spoken of his interest in facing the original Mondasian Cybermen from 1966's The Tenth Planet, so Moffat decided that Bill would be transformed into one of these early Cybermen.

Steven Moffat felt that Missy's redemption had to have a tragic end, with an earlier Master as the architect of her downfall

As the year wore on, the latter part of Season Thirty-Six came to be focussed on the possibility that Missy would make amends for her past misdeeds. This putative change of heart came in the wake of the Doctor's decision to sacrifice his own freedom to serve as her guardian, rather than allow her to be executed, as established in episode six, Extremis. Moffat felt that Missy's redemption had to have a tragic end, and he decided that this would most appropriately come to pass if an earlier incarnation of the Master was the architect of her downfall.

As far back as 2009, John Simm -- who had played the Master opposite David Tennant's Tenth Doctor from 2007 to 2010 -- had assured Moffat of his openness to return to the role. Indeed, Simm was hoping to be involved in the fiftieth-anniversary special, The Day Of The Doctor, during 2013. Moffat was keen to have an actor of Simm's calibre back on Doctor Who but, having introduced Gomez as Missy during Season Thirty-Four, he knew he would have to delay Simm's appearance to avoid undermining the new incarnation. As such, the Season Thirty-Six finale presented an ideal opportunity to bring the two Masters together -- a plot twist that had never previously been exploited on television.

For the story's setting, Moffat conceived of a colony ship where the passengers became trapped for generations following the malfunction of the robotic pilot. This notion was refined through conversations with his son, Joshua, who was studying Physics at university. The younger Moffat had developed an interest in relativistic effects: the way that normal physical laws broke down at speeds approaching that of light, or in the presence of an enormous mass. One associated phenomenon was time dilation, which saw time passing at different rates for different observers, and he noted his surprise that it featured so rarely in popular science-fiction.

This discussion prompted Moffat to replace the ship's robotic pilot with a black hole -- an extinct star so massive that even light could not escape the pull of its gravitational attraction -- which would cause time to accelerate on the parts of the vessel that were closest to it. This device would also provide the opportunity to incorporate more recent types of Cybermen. Moffat was less convinced than Capaldi about the effectiveness of the Mondasian versions, but the rapid passage of time would allow them to evolve into the forms introduced in 2006's Rise Of The Cybermen and 2013's Nightmare In Silver.

Steven Moffat had assumed that the 2017 Christmas special would be the Thirteenth Doctor's debut adventure

By the start of 2017, a significant change to the finale had been agreed upon. It was now known that Chibnall wanted the Thirteenth Doctor's first adventure to be the Season Thirty-Seven premiere, rather than that year's Christmas special. Not only did this mean that Doctor Who might be absent from the BBC's holiday schedule for the first time in more than a decade, but it would also preclude the now-traditional appearance of the new Doctor in the immediate aftermath of her predecessor's regeneration. Under the circumstances, Moffat, Minchin and Capaldi all committed to recording an additional episode -- ultimately, Twice Upon A Time -- during the summer. While the Season Thirty-Six finale would still lay the groundwork for the regeneration, the event itself would now be the culmination of the Christmas broadcast.

Moffat completed a draft of episode eleven, the season finale's opening installment, in mid-January. An influence on the script's tone was his mother's hospitalisation during the latter part of 2016, which sadly prefigured her death at the end of the year. These events led Moffat to conceive the eerie institution in which Bill found herself after awakening on Floor 1056. Missy's assertion that the Doctor once called himself “Doctor Who” was a reference to the longstanding debate over whether Doctor Who was just the title of the programme, as most fans traditionally asserted, or also the name of its main character, as was often assumed by the general public. Indeed, over the years, there had been occasional evidence within the show that the Doctor did indeed call himself Doctor Who, although fans typically dismissed such incidents as production errors. In these scenes, Jorj was briefly the more prosaic George. The proto-Cybermen carrying drip feeds were originally dubbed Top Knots, in reference to their balaclava-like head coverings, although they would later simply be known as Patients. Initially, the closing zoom on Bill's eye was to reveal the appearance of a star-like defect, the same abnormality that Heather had possessed in The Pilot.

The first draft of the concluding episode was finished about a month later. It employed Moffat's preferred device for two-part stories, by pivoting to a very different principal setting than the opening installment. Whereas episode eleven had been mainly set in gloomy, claustrophobic environments, episode twelve would chiefly take place at a solar farm which would appear to be situated amidst rural fields and woods. The notion of Bill principally appearing on-camera in human form -- to reflect the way that she still saw herself -- was influenced by the American science-fiction series Quantum Leap, in which the same approach was employed for body-hopping time traveller Dr Sam Beckett. The farm's location on Floor 507 was a tribute to Moffat's predecessor, Russell T Davies, who often included the number 57 in his scripts; for instance, this was the number of years which the Doctor asserted Rose Tyler had aged in 2006's Doomsday when he was feigning that her mother, Jackie, was actually his companion.

Comic strip writer Grant Morrison had posited that the evil Voord of The Keys Of Marinus evolved into the Cybermen

The Doctor's list of worlds that had begotten Cybermen referenced various planets that had become associated with them over the years. In addition to Mondas, Telos was where their tombs were discovered in 1967's The Tomb Of The Cybermen, the Cyber Director of 1968's The Invasion recognised the Second Doctor from an encounter on Planet Fourteen, and an alternative Earth had seen Cybus Industries spawn a new race of Cybermen in 2006's Rise Of The Cybermen / The Age Of Steel. More obscurely, Marinus was the home of the evil Voord in 1964's The Keys Of Marinus. The World Shapers, a 1987 Doctor Who Magazine comic strip by Grant Morrison, had posited that the Voord evolved into the Cybermen.

The First Doctor's involvement in the closing sequence -- a cliffhanger leading into Twice Upon A Time -- was inspired by comments Capaldi had made at the New York Comic Con on October 7th, when he suggested that actor David Bradley could play the Time Lord's original incarnation. In addition to his portrayal of the villainous Solomon in 2012's Dinosaurs On A Spaceship, Bradley had also taken the role of William Hartnell, the original star of Doctor Who, in the following year's docudrama An Adventure In Space And Time.

The two-part finale was made as the seventh and final production block for Season Thirty-Six. Moffat was keen that director Rachel Talalay should return for the story. She already had experience of Missy and the Cybermen from Season Thirty-Four's concluding adventure, Dark Water / Death In Heaven, after which she had made Heaven Sent and Hell Bent, the last two installments of Season Thirty-Five. Returning to Doctor Who after an absence of eight years, Simm decided that he would portray a distinctly older version of his Master -- in part because his hair had gone grey in the interim. He also opted to grow a goatee, in homage to earlier Master actors Roger Delgado and Anthony Ainley; Simm had previously considered sporting similar facial hair for his initial appearances as the Master, before he and Davies had decided that it was unnecessary.

The first day of production on the finale was February 24th, when Talalay began recording scenes on the set for Mr Razor's nest at Roath Lock Studios in Cardiff. This work continued after the weekend on the 27th, alongside the shots of Jorj in the room off the bridge. The rest of the week -- from February 28th to March 3rd -- was chiefly devoted to action on the bridge itself. For the Doctor's use of Venusian aikido, Capaldi studied footage of Jon Pertwee in an attempt to recreate the Third Doctor's movements when applying the extraterrestrial martial art. Prior to a break for the weekend, Talalay also spent part of the 3rd on sequences in the lift.

Work on March 6th started at Roath Lock, on the set for the hospital ward. That evening, Bute Street was the venue for Bill and Mr Razor on the streets of Floor 1056. Pick-up and effects shots were also taped both in the studio and on location. On March 7th, Cardiff University posed as St Luke's University for the final time. It was back to Roath Lock for the rest of the week, with Talalay focussing on hospital scenes -- including the wards, the surgical theatre and the corridors -- from the 8th to the 10th. Part of the last day was also spent on material in the TARDIS.

Filming during the following week was entirely confined to Roath Lock. More sequences in the surgical theatre were taped on March 13th and 14th, alongside additional footage in the lift on the first day and a start on action atop the hospital roof on the second day. The rooftop continued to be Talalay's primary concern through to the 17th. The latter day also saw more recording on the ward set, plus those for the farmhouse cellar and the adjacent service duct, as well as the completion of a number of pick-up and effects shots. The production then stood down for another weekend off.

The venue for the week of March 20th to 24th was Llanvetherine Court in the village of Llanvetherine, where recording was augmented with the use of an aerial drone. The location was chosen because it offered a wooden barn which suited Talalay's vision of Floor 507 -- and, briefly on the first day, Floor 502 -- as being akin to the homesteads of the American west during the early twentieth century. Temporary modifications were correspondingly made to the Georgian farmhouse. March 27th, the first day after the weekend, was also spent at Llanvetherine. It was back to Roath Lock on the 28th, when Talalay's itinerary included Nardole and the evil Time Lords in the cockpit of the shuttlecraft, Missy and the Master in the upstairs room of the farmhouse, and various inserts. Footage of Bill and Mr Razor on the streets of Floor 1056 was also captured with the help of a green screen. From March 29th to 31st, the grounds of MOD Caerwent served as the woods on Floor 507.

The prologue and epilogue in the Antarctic wastes were recorded during production on Twice Upon A Time

Following the weekend, the final day at MOD Caerwent was April 3rd. Part of the day was devoted to the aftermath of the enormous explosion, with Floor 507 now a charred wasteland. An additional day at Llanvetherine Court was scheduled for the 4th, to capture the remaining shots of both Floor 507 and Floor 502. Next, the farmhouse kitchen was represented as a studio set on April 5th and 6th. Material in the dormitory and the farmhouse hallway was also taped on the first day, and more inserts were on the schedule for the second day. The last day of principal photography on the finale was April 7th, when Talalay completed the concluding TARDIS scene. However, work on the two episodes wasn't quite finished. The prologue and epilogue sequences in the Antarctic wastes were recorded at Roath Lock on June 12th, during production on Twice Upon A Time. Talalay also seized the opportunity to tape some additional pick-up shots in the studio on June 14th.

A key sequence assembled in post-production was the start of the Doctor's regeneration. The montage of the Doctor's companions and associates uttering his name was inspired by a similar scene in 1981's Logopolis, the last story for Tom Baker's Fourth Doctor. Moffat had allowed for the possibility that clips from twentieth-century Doctor Who might be included, but only excerpts which postdated the programme's 2005 revival were ultimately used. They featured Bill (The Pilot and The Lie Of The Land), Nardole (Oxygen), Rose Tyler (Bad Wolf), Martha Jones (Daleks In Manhattan), Donna Noble (Partners In Crime), Jack Harkness (Utopia), Madame Vastra (The Name Of The Doctor), Jenny Flint (The Crimson Horror), Sarah Jane Smith (The Stolen Earth), Amy Pond (The Impossible Astronaut), Clara Oswald (Last Christmas), River Song (The Pandorica Opens) and Missy (The Witch's Familiar).

The first episode was ultimately named World Enough And Time, a title drawn from the opening line of Andrew Marvell's To His Coy Mistress, published posthumously in 1681. The poem represented an entreaty for a woman to respond favourably to the speaker's romantic advances, contrasting the lengths to which he would go if he were unencumbered by time and space, with the urgency imposed by the true brevity of the human lifespan. The concluding installment, The Doctor Falls, was accorded an hour-long timeslot. Amongst dialogue trimmed from the episode was Nardole's revelation to Hazran that he had wiped all memory of the first twenty years of his life, for reasons that he could no longer recall. The timeframe to complete the transmission version of the finale was extremely tight, with changes still being made to The Doctor Falls in the week preceding its broadcast.

On May 16th, Michelle Gomez confirmed to the Digital Spy website that Season 36 would be her last on Doctor Who

On May 16th, Gomez confirmed to the Digital Spy website that Season Thirty-Six would be her last on Doctor Who. World Enough And Time aired on June 24th, in the same 6.45pm timeslot as The Eaters Of Light the week before. On the other hand, the extended length of The Doctor Falls saw it scheduled at 6.30pm on July 1st. In the weeks that followed, the two shows which had been airing before and after Doctor Who -- Pointless Celebrities and Pitch Battle -- were pushed together to fill the gap which resulted from the end of Season Thirty-Six.

Later the same month, on July 16th, a special video aired immediately after the conclusion of BBC One's coverage of the Wimbledon Gentlemen's Final. It revealed that Capaldi's successor was none other than Jodie Whittaker, whose Thirteenth Doctor would be the Time Lord's first female incarnation. But while the countdown was now on to the next era of Doctor Who, there was still one more story to tell about the Twelfth Doctor -- not to mention his very first self -- at Christmas...

• Doctor Who: The Complete History #89, 2019, “Story 275: World Enough And Time / The Doctor Falls”, edited by Mark Wright, Hachette Partworks Ltd.