In the year 5343, a mistake by River Song's bumbling accomplice, Nardole, embroils the Doctor in her scheme to recover a priceless diamond. Known as the Halassi Androvar, it is embedded in the brain of Hydroflax, a galactic tyrant whose disembodied head pilots a powerful robot. Not only does River appear to be unaware of the Doctor's true identity, but she claims to be Hydroflax's wife -- as well as the bride of Ramone, one of her co-conspirators. The heist leads the Doctor and River to a spaceship full of the universe's vilest denizens, and a date with destiny that the Doctor has been avoiding for a very long time.

Steven Moffat had agreed to succeed Russell T Davies as Doctor Who's executive producer in 2007, with his first episodes to be broadcast in 2010. At the time, he envisaged staying in the post for three seasons, exiting around the time of the programme's fiftieth anniversary in November 2013. However, the pressure to develop the milestone special that became The Day Of The Doctor -- while also contending with the near-simultaneous departure of the Eleventh Doctor, Matt Smith -- had taken its toll on Moffat. He decided that he did not want to leave Doctor Who on a sour note, and he consequently agreed to remain on the programme to usher in Peter Capaldi's Twelfth Doctor, starting with Season Thirty-Four in 2014. Moffat was also keen to finish the story of companion Clara Oswald, which wound up continuing into Season Thirty-Five after actress Jenna Coleman made a late decision to extend her stay by a year.

Finally, as work on Season Thirty-Five neared completion in mid-2015, Moffat believed that the end of his Doctor Who tenure was in sight. Coleman was leaving in the season finale, Hell Bent, while his likely successor had emerged in the form of Chris Chibnall. A lifelong Doctor Who fan, Chibnall had written five scripts for the programme -- most recently 2012's The Power Of Three -- and he had also been a lead writer and co-producer on the spin-off series Torchwood. He now had substantial showrunning experience on programmes such as Law & Order: UK, and he had recently scored a massive popular success with the mystery drama Broadchurch starring the former Tenth Doctor, David Tennant. As such, Moffat decided to develop the 2015 Christmas special as his Doctor Who swansong.

Last Christmas had turned out to be darker than intended, so Steven Moffat now wanted to write a comedic adventure

Moffat's previous yuletide script, Last Christmas, had turned out to be a rather darker narrative than he had originally intended. He now wanted to write a much more comedic adventure, which would also serve as a tonic for the Doctor's vengeful, selfish disposition following Clara's apparent death during Season Thirty-Five. Encouraged by Davies, Moffat decided to pair the Twelfth Doctor with River Song, the time-travelling archaeologist he had created for 2008's Silence In The Library / Forest Of The Dead. Played by Alex Kingston, River had been a key figure during the Eleventh Doctor's era -- arguably even marrying the Doctor in 2011's The Wedding Of River Song -- but her appearance in 2013's The Name Of The Doctor was heavily implied to be her last.

Fortunately, River's meetings with the Doctor had always been told out of chronological order. Indeed, she had been killed in her first appearance, which was said to have taken place after a trip with the Doctor to see the Singing Towers of Darillium. With the exception of The Name Of The Doctor, in which she was a data ghost emanating from a time after her death, all of River's other appearances had chronicled various earlier stages of her life. Moffat now realised that it would be appropriate to end his tenure on Doctor Who by depicting the visit to Darillium, since Silence In The Library / Forest Of The Dead had been the last story he had written before becoming executive producer. River was also the perfect character to include in the romantic comedy that he was envisaging for Christmas. Kingston was delighted to confirm her availability for the recording dates; since completing her scenes for The Name Of The Doctor in November 2012, she had begun recording a series of audio plays for Big Finish Productions under the banner The Diary Of River Song, having assumed that her involvement with the television programme was at its end.

Moffat began writing his script in late July, and it was completed around the middle of August. At this stage, the diamond was called the Kalezzi Androvar. The Doctor's reference to having had a haircut and donning his best suit was an allusion to River's description in Forest Of The Dead of his appearance during their date on Darillium. Meanwhile, Moffat had met with Chibnall for dinner with the intention of offering him the job of Doctor Who's executive producer. Over the course of the meal, however, he learned that Chibnall was deeply immersed in developing the third season of Broadchurch, which would enter production in early 2016. As a result, Moffat decided to delay making the invitation, and he elected to stay on Doctor Who for one more season.

Douglas Mackinnon would be prominently involved with BBC One's Christmas and New Year's schedules

The 2015 Christmas special entered production immediately after the completion of Season Thirty-Five, effectively making it the year's ninth recording block. The director was Douglas Mackinnon, who had most recently handled Season Thirty-Four's Flatline. Mackinnon was delighted to be working on the special, because it meant that he would be prominently involved with both the Christmas and New Year's schedules on BBC One; the latter project was The Abominable Bride, a special edition of Sherlock on which Moffat was also an executive producer. For the appearance of Hydroflax's spaceship, Mackinnon wanted to evoke the stereotypical image of a flying saucer -- as popularised by movies like 1956's Forbidden Planet -- which he thought resembled a traditional Christmas tree decoration.

Cast in the role of Nardole was Matt Lucas, who was best known as the co-creator of the popular comedy programme Little Britain alongside David Walliams, who had played Gibbis in 2011's The God Complex. Lucas had a prior involvement with Doctor Who in the form of The One Doctor, a Sixth Doctor audio drama released by Big Finish Productions in 2001. He had subsequently become better-acquainted with the programme via his ex-husband, Kevin McGee, who had been a big Doctor Who fan. However, McGee's suicide in 2009 left Lucas with complex feelings towards the show; fortunately, he already had a good working relationship with Capaldi from their time making the 2014 family film Paddington.

The first day of recording was September 1st, with the cameras rolling on the standing TARDIS set at Roath Lock Studios in Cardiff. Mackinnon also captured a shot of the police box roof, which would be used for special effects. The 2nd was spent across the English border in Bristol, where the Leigh Court Business Centre, housed in a nineteenth-century manor house, offered spaces suitable for the storeroom, lobby and office aboard the starliner Harmony And Redemption. It was back to Roath Lock from September 3rd to 5th, where the focus was the street on Mendorax Dellora on which the TARDIS had materialised; this set was repurposed from the trap street which had figured prominently in Season Thirty-Five's Face The Raven. Work on the 5th wrapped after a start on sequences in Hydroflax's throne room.

Following a day off on Sunday the 6th, the remaining throne room scenes were completed on September 7th and 8th. The latter day then saw Mackinnon's attention shift to the flying saucer's ready room, where filming continued onto the 9th. A second day at the Leigh Court Business Centre was scheduled for September 10th, to shoot more action in the starliner lobby. The 11th was again confined to the studio, with Mackinnon finishing the remaining ready room scenes before moving over to the TARDIS console room, prior to a break for the weekend.

Roath Lock remained the centre of activity on September 14th and 15th. Material in and around the hold was on the itinerary for the first day, followed by various inserts on the second day. On the 16th, Mackinnon's team relocated to MOD St Athan, where the snowy field around Hydroflax's spaceship had been constructed inside a hangar. Modifications were then made on the 17th so that the same venue could represent the site to which Ramone teleported the Doctor and River. Once this material was completed, cast and crew travelled to Aberthaw Cement Works in East Aberthaw, which served as the Harmony And Redemption crash site on Darillium. Lucas had now finished his work on the adventure, and he had thoroughly enjoyed the experience. In fact, his next project would again see him work with many of the same crew, as Russell T Davies' adaptation of William Shakespeare's A Midsummer Night's Dream entered production at Roath Lock while Doctor Who was on hiatus. Back in the studio on September 18th, Mackinnon made a start on the dining room scenes aboard the starliner before another pause for the weekend.

The final week of production on the Christmas special took place almost entirely at Roath Lock. Material in the Harmony And Redemption dining room was completed across September 21st to 23rd, with Capaldi persevering despite a stomach ailment. Mackinnon spent the latter part of the 23rd on the starliner's bridge, while a number of inserts and effects shots were also taped. The remaining sequences on the bridge were recorded on September 24th, after which filming began on the balcony overlooking the Singing Towers. This work continued onto the 25th, with Mackinnon then moving to the reception area of Alphonse's restaurant; additional inserts were also taped. The same day, at a Radio Times event, Moffat confirmed that Doctor Who would be continuing to its thirty-sixth season, and that he would be remaining with the programme.

Finally, on September 26th, more TARDIS material was taped at Roath Lock alongside further pick-up shots. Meanwhile, a brief location excursion saw the Doctor check on Alphonse's progress at the Capital Quarter building in Cardiff, which was then under construction. This sequence was ultimately deleted in editing, and would have seen the Doctor advise an older Alphonse to adjust his architectural plans for the restaurant to include a balcony with a view of the Singing Towers.

Peter Capaldi underwent surgery to repair the knee injury he had suffered during filming in May

Having reached the end of the year's work on Doctor Who, Capaldi underwent surgery to repair the knee injury he had suffered while filming The Zygon Invasion / The Zygon Inversion in May. In post-production, the visual inspiration for the Singing Towers was the Old Man of Storr, a rocky prominence on Mackinnon's native Isle of Skye which was said to be formed from the fingers of a giant who perished and was buried there. The special was still untitled by November 12th, when Moffat discussed the matter with Doctor Who Magazine editor Tom Spilsbury. He ultimately opted for The Husbands Of River Song over the singular “The Husband Of River Song” -- although they also jokingly contemplated the title “Bigamy In Space”!

The Husbands Of River Song was broadcast on December 25th, as was now traditional for a Doctor Who Christmas special. It also enjoyed a limited run in cinemas on December 28th and 29th, with screenings in Austria, Canada, Denmark, Germany and the United States. Doctor Who then entered its longest transmission hiatus since returning to television in 2005. With Season Thirty-Six deferred to the spring of 2017, the next episode to air would actually be the 2016 Christmas special, The Return Of Doctor Mysterio, exactly one year later. In the interim, a short-lived spin-off series called Class would be screened, with Capaldi making an appearance as the Doctor in its first episode, For Tonight We Might Die, on October 22nd.

• Doctor Who: The Complete History #84, 2018, “Story 263: The Husbands Of River Song”, edited by Mark Wright, Hachette Partworks Ltd.