On the verge of regenerating but weary of his long life, the Doctor finds himself in 1986 Antarctica. There he comes face to face with his original self, who is contemplating death as an alternative to enduring his first regeneration. Time suddenly comes to a stop, and they meet a British army captain from the First World War who is being pursued by a mysterious glass woman. Taken aboard her spaceship, the three men arrive in the Chamber of the Dead where, impossibly, the Doctor is reunited with Bill Potts. To understand the truth of these events, they embark on what may be the Doctor's final journey into time, space and memory.

Steven Moffat had nearly exited Doctor Who on three occasions before he finally wrapped up his tenure as executive producer on the 2017 Christmas special. He had originally meant to stay for only three seasons, with 2013's The Time Of The Doctor a possible departure point. But that year's fiftieth-anniversary special, The Day Of The Doctor, had presented a gruelling challenge, and Moffat did not want to leave Doctor Who on what he feared would be a sour note. Furthermore, once Peter Capaldi agreed to become the Twelfth Doctor with Season Thirty-Four in 2014, Moffat was eager to work with the esteemed actor. He instead planned to wind down his involvement with the show as of the 2015 Christmas special, The Husbands Of River Song. But his intended successor, Chris Chibnall, still had commitments to the final season of the mystery drama Broadchurch, prompting Moffat to stay for one more year.

Throughout 2016, Moffat expected that the Season Thirty-Six finale, World Enough And Time / The Doctor Falls, would be the swansong not only for himself but also for his fellow executive producer, Brian Minchin, and their entire regular cast of Peter Capaldi, Pearl Mackie and Matt Lucas. Chibnall had assured Capaldi that he was welcome to remain as the Doctor, but the series star had come to feel that it was time to explore new professional opportunities. Moffat had intended all along that Bill and Nardole would be written out at the end of the year, so that Chibnall could introduce companions of his own. Consequently, The Doctor Falls was structured to end with the Doctor, alone, undergoing his latest regeneration after finally succumbing to injuries inflicted by the Cybermen. Moffat assumed that the 2017 Christmas special would then represent the first adventure for the Thirteenth Doctor. He preferred this schedule, because he felt that the holidays were the wrong time of year for a regeneration -- especially since he had already overseen the transition between the Eleventh and Twelfth Doctors in The Time Of The Doctor.

Steven Moffat, Brian Minchin and Peter Capaldi all agreed to make an additional Doctor Who adventure

Around the start of 2017, however, Moffat learned that Chibnall would not be producing an episode for that year's festive season: the incoming showrunner wanted to launch the Thirteenth Doctor with a full run of stories, rather than an isolated special. Furthermore, there was no guarantee that the new series star would be cast prior to the end of production on Season Thirty-Six. Faced with the possibility of Doctor Who's absence from the Christmas schedule, and that Capaldi might be deprived of an on-screen handover to his successor, Moffat, Minchin and Capaldi all agreed to make an additional adventure during the summer. The Doctor Falls would still set up the circumstances of the regeneration, but the Christmas special would now act as an extended coda, with the Doctor delaying a metamorphosis that he was no longer convinced he wanted to endure.

As a narrative hook, Moffat decided to pursue an idea that he had discussed during the New York Comic Con on October 7th. At the time, he had opined that he would love to be able to unite the Twelfth Doctor with his original incarnation, as seen at the very start of Doctor Who in 1963. The earliest presentation of the First Doctor had been notably anti-heroic, with the character just as often posing problems and causing conflicts as resolving them. As such, Moffat thought that it would be interesting for that version of the Doctor to see what he had become after so many lives. When he had observed that this notion was sadly impossible to realise due to the fact that William Hartnell, the actor who played the First Doctor, had died in 1975, Capaldi quipped that they should instead offer the role to David Bradley, who had portrayed Hartnell in the 2013 docudrama An Adventure In Space And Time.

By coincidence, the Mondasian Cybermen whom the Doctor had encountered in Hartnell's last story, 1966's The Tenth Planet, would feature prominently in World Enough And Time / The Doctor Falls. The existence of the appropriate costumes, together with the theme of surrendering to the process of regeneration, suggested that the Christmas special should feature the First Doctor at the end of his life, rather than at the beginning. Episode Four of The Tenth Planet saw the Doctor separated from companions Ben Jackson and Polly as he made his way back to the TARDIS, and Moffat imagined that this was the window of time in which he would encounter the Twelfth Doctor. Although Hartnell's character had softened considerably during his years in the role, Moffat nonetheless wanted to reflect the received wisdom that he was a crotchety, irascible old man. This would also provide an opportunity to poke fun at some of the more antiquated elements of Sixties Doctor Who, such as the condescending and chauvinistic attitudes that it sometimes adopted towards female characters, even though the Doctor himself had rarely exhibited these traits.

Chris Chibnall would script the Thirteenth Doctor's appearance in the Christmas special

During March, Moffat met with Chibnall and they agreed that, as had been the case when Moffat took over from Russell T Davies for the closing moments of 2010's The End of Time, Chibnall would script the Thirteenth Doctor's appearance during the last seconds of the Christmas special. Around the same time, Rachel Talalay -- who was in the process of directing World Enough And Time / The Doctor Falls -- was asked to handle Capaldi's valedictory adventure as well. Talalay, who made her home in Canada, was initially uncertain about a protracted stay in the United Kingdom, especially since she was in need of surgery to remove her ailing gall bladder. However, she ultimately found herself unable to refuse the assignment.

Given the weighty subject matter in which not one but two Doctors would be contemplating the end of their lives, Moffat sought as many opportunities for levity as possible. The contentious relationship between the First and Twelfth Doctors -- echoing the dynamics between different incarnations as far back as 1972's The Three Doctors -- would drive much of the story's humour. Moffat also decided that there would be no actual villain; instead, the plot would hinge on a mistake by the far-future Testimony Foundation, which was inspired by Moffat's view that memories were a person's essential building blocks.

Furthermore, the Doctors would be paired with two companion figures. One would be Bill Potts, who would be revealed to be a Testimony avatar with all of the companion's memories. In June, Mackie was surprised and delighted to receive a message from Moffat asking if she was available to return to Doctor Who, since she had assumed that her time on the programme had ended with the completion of Season Thirty-Six.

The Doctors would also be accompanied by the Captain, a soldier who would be revealed as the ancestor of Brigadier Lethbridge-Stewart, the beloved character played by Nicholas Courtney on a recurring basis from 1968's The Web Of Fear until his death in 2011. Moffat knew that his affection for the Brigadier was shared by Mark Gatiss, whose numerous Doctor Who scripts had most recently included Season Thirty-Six's Empress Of Mars. Gatiss was also an actor; indeed, he had played Professor Lazarus in 2007's The Lazarus Experiment, went uncredited as the voice of a pilot in 2010's Victory Of The Daleks and 2011's A Good Man Goes To War, and was concealed behind prosthetics and a stage name to portray the alien Gantok in 2011's The Wedding Of River Song. Moffat wanted his friend and frequent collaborator to participate in his final Doctor Who episode, and so the Captain was conceived with Gatiss in mind.

The inclusion of the Captain suggested the First World War as a setting for the Christmas special

The inclusion of the Captain suggested the First World War as a setting for the Christmas special. This led Moffat to situate some of the action during yuletide in 1914, five months into the conflict, when widespread ceasefires along the Western Front paused the hostilities for a few days. The truce facilitated exchanges of prisoners and the recovery of the dead from the No Man's Land between the trenches, and it also witnessed the Allied soldiers of the United Kingdom and France trade gifts with their German counterparts, sing Christmas carols together, and even compete in friendly football games. Moffat had considered featuring these events in Doctor Who on previous occasions, but he had demurred as he searched for a way to do so in a respectful manner.

Moffat completed his first draft of the script for the Christmas special -- eventually called Twice Upon A Time -- in late May. Originally, the Twelfth Doctor revealed that Ben and Polly had gotten married after leaving the TARDIS, and were now running a pub in Weston-super-Mare, a town in Somerset. The inclusion of the Weapon Forges of Villengard hearkened back to Moffat's first Doctor Who script, 2005's The Empty Child / The Doctor Dances, in which the Doctor claimed responsibility for their destruction. Chibnall had now delivered his pages; they were written in a manner which made no reference to the Thirteenth Doctor's sex, but Moffat had come to suspect that the new series star would be female. Indeed, Moffat himself had helped paved the way for the Doctor's potential change of gender by introducing Missy, a female regeneration of the evil Time Lord otherwise known as the Master, whose previous incarnations had been exclusively male.

In addition to Mackie, Moffat hoped that the Twelfth Doctor's other companions -- Jenna Coleman as Clara, and Matt Lucas as Nardole -- would also appear in the special's closing scene. Both actors confirmed their interest, although Coleman's availability was limited due to her commitments to the second season of the historical drama Victoria. Chibnall asked Moffat to incorporate the destruction of the TARDIS console room into Twice Upon A Time, since he was planning to introduce a new standing set for the Thirteenth Doctor.

After receiving the script, Bradley decided to avoid the sort of outright impression of Hartnell that he had attempted for An Adventure In Space And Time. Meanwhile, another Doctor Who writer had joined Twice Upon A Time with a role in front of the camera. This was Toby Whithouse, whose most recent script for the series had been Season Thirty-Six's The Lie Of The Land, and who would now be playing the unnamed German soldier. Although he had never appeared in Doctor Who before, Whithouse was an established actor; indeed, at the time, he was staging a one-man show called Executioner Number One. Replacing the late Michael Craze as Ben was former Hollyoaks regular Jared Garfield, while Anneke Wills would be succeeded as Polly by Lily Travers, whose previous roles included the feature films Kingsman: The Secret Service and Me Before You.

David Bradley wore the Karakul hat he had sported in An Adventure In Space And Time

Filming for Twice Upon A Time began on June 12th, two months after work had wrapped on Season Thirty-Six. The first order of business was the material in the Antarctic wastes, since the set would also be needed for throw-forward material in World Enough And Time / The Doctor Falls, which was scheduled for transmission just twelve days later. These scenes were recorded at Doctor Who's regular production home, Roath Lock Studios in Cardiff, through to the 14th. The First Doctor's TARDIS and costume were both new for the special, although Bradley asked to wear the Karakul hat he had sported in An Adventure In Space And Time. June 15th and 16th were spent in the Twelfth Doctor's TARDIS console room, prior to a break for the weekend.

The following week was also confined to Roath Lock, with Talalay concentrating on sequences in the Chamber of the Dead from June 19th to 21st. Part of the latter day saw the cameras move over to the throne room set. This work was completed on the 22nd, alongside footage of the Testimony Foundation's white room, and the TARDIS scene of the Doctor and the Captain heading back to 1914 Ypres. Filming in the First Doctor's TARDIS console room began on June 23rd. As with the old-style TARDIS seen in Season Thirty-Five's Hell Bent, it was represented by a modified version of the set constructed for An Adventure In Space And Time, which was transported to Roath Lock from its regular home at the Doctor Who Experience, a standing exhibition on Cardiff Bay that was scheduled to close a few weeks later. The set's use for Hell Bent had been marred by a measurement gaffe, which resulted in the manufacture of an undersized outer door; this error was now corrected. Also constructed for Twice Upon A Time was a replica of the astral map originally seen in 1965's The Web Planet.

Following the weekend, the remaining scenes in the First Doctor's TARDIS were taped on June 26th. The rest of the week -- from the 27th to the 30th -- was spent at Dragon Studios in Llanilid, where the Ypres battlefield was staged. The recreation of the Christmas armistice proved to be an emotional experience for the cast and crew, especially during the singing of Silent Night and its German antecedent, Stille Nacht, heilige Nacht. Lucas was present on the 30th to film his last performance as Nardole. The cuddle between the Doctor, Bill and Nardole was ad-libbed, and referenced a similar group hug in Season Thirty-Six's Oxygen. Unfortunately, Coleman was not able to be present on location, as a result of which Moffat revised Clara's appearance so that it could be recorded in the studio against a green screen.

July 7th marked Pearl Mackie's final day of work on Doctor Who

Another week at Roath Lock began on July 3rd, when Talalay captured Helen Clay's presentation about the Testimony Foundation, plus recreations of scenes from The Tenth Planet set on the Cyberman spaceship and in Snowcap Base. The main focus from the 4th to the 7th were the ruins of Villengard. Part of July 6th was also spent on shots of the Doctors, Bill and the Captain escaping down the massive chains, while additional work on the 7th included various inserts, plus the Doctor's encounter with Rusty in the tower. The Dalek was represented by the same casing used for Rusty's original appearance in Season Thirty-Four's Into The Dalek. It had been on display at the Doctor Who Experience during the intervening years, and received only minor modifications for its inclusion in Twice Upon A Time. July 7th marked Mackie's final day of work on Doctor Who.

Meanwhile, Jodie Whittaker, one of the stars of Chibnall's Broadchurch, had been cast as the Thirteenth Doctor. On July 9th, while the cast and crew of Twice Upon A Time were enjoying their weekend off, director Jamie Childs undertook a special recording session for the revelation of the new series star's identity. Capaldi's final day as the Twelfth Doctor was July 10th. In addition to some pick-up shots, Talalay's itinerary included material in the tower room and on the stairs which led to it, as well as the TARDIS sequence which culminated in the regeneration. Although he had now recorded all of his scenes as the Captain, Gatiss returned to the set to surprise Capaldi with a jacket worn by the Third Doctor, Jon Pertwee, in 1973's Planet Of The Daleks. Seizing the opportunity, Talalay decided to place it in the background during the regeneration.

On July 15th, Doctor Who returned to its old home at Television Centre in White City, London, which was now owned by the commercial subsidiary BBC Studioworks. This was where Coleman recorded her cameo appearance as Clara, on a sound stage normally used by the automotive programme Top Gear. The next day, Whittaker's casting was announced via the video which had been taped on the 9th. Running to sixty seconds, it was broadcast immediately after the Wimbledon Gentlemen's Final on BBC One. Talalay then filmed Whittaker's contribution to Twice Upon A Time at Roath Lock on July 19th and 20th.

Rachel Talalay decided to make only minimal use of the recreated sequences from The Tenth Planet

In post-production, Talalay decided to make only minimal use of the recreated sequences from The Tenth Planet, since she felt that the Sixties production aesthetic had translated poorly to high-definition colour. In the ruins of Villengard, the Doctors originally faced landmines and tentacled creatures before they reached the tower. In the process, the First Doctor used the Twelfth Doctor's sonic screwdriver to rescue his future self from the monsters. The Twelfth Doctor then remembered that this was the first time he had ever used a sonic screwdriver, a device originally introduced as a tool owned by the Second Doctor in 1968's Fury From The Deep. The caption at the start of Twice Upon A Time initially referred to Episode Four of The Tenth Planet as being 705 episodes ago, rather than 709, but Doctor Who Magazine editor Tom Spilsbury caught the error shortly before the story's broadcast on Christmas Day.

Over the next few days, Twice Upon A Time was also screened in movie theatres in Brazil, Australia, Denmark, Canada and the United States. And already, anticipation was running high for Season Thirty-Seven, which would be broadcast in the autumn of 2018. While a loud minority had criticised the introduction of a female Doctor, the unprecedented development had stoked substantial new interest in Doctor Who. As the Thirteenth Doctor faded from view, tumbling towards the Earth far below, the only question that remained was whether Chibnall and Whittaker could stick the landing...

• Doctor Who: The Complete History #89, 2019, “Story 276: Twice Upon A Time”, edited by Mark Wright, Hachette Partworks Ltd.