The Doctor accidentally bestows the powers of a comic book superhero on a young boy named Grant. Grant promises that he will hide his extraordinary abilities but, as an adult, he reneges on his oath and becomes a masked vigilante known as the Ghost. Grant and the Doctor meet again when they both come to the aid of investigative journalist Lucy Fletcher. She has discovered that Harmony Shoal, an international research and development company, is really a front for an invasion of Earth. But the brain-like creatures behind Harmony Shoal have the ability to possess the human form -- and Grant becomes their next target.

For some time, Steven Moffat had believed that Doctor Who's 2015 Christmas special, The Husbands Of River Song, would be his final involvement with the programme. When his intended successor, Chris Chibnall, proved unavailable for the programme's thirty-sixth season -- which was scheduled for broadcast in the spring of 2017 -- Moffat agreed to remain on the show for an additional year. Although this would mean that he would also be responsible for a 2016 Christmas special, his initial focus was the more substantial task of setting up Season Thirty-Six. In the course of this work, during early 2016, one of the ideas he discussed with his fellow executive producer, Brian Minchin, was the notion of the Doctor meeting a superhero. Minchin liked the concept, and he suggested that it was ideal subject matter for the holiday episode.

Although superheroes were closely related to earlier masked adventurers like Zorro, the new genre emerged in 1938 when Jerry Siegel and Joe Shuster introduced Superman in the pages of Action Comics. Hundreds of imitators quickly followed, and superheroes soon migrated to other media. However, the technological limitations of early visual effects meant that superhero movies and television shows presented enormous production challenges. Prior to the turn of the century, rare successes included the beloved Superman films starring Christopher Reeve which debuted in 1978, two versions of Batman -- a campy Sixties show with Adam West and a more brooding film series launched in 1989 -- and a handful of low-tech television adaptations like The Incredible Hulk and Wonder Woman. This situation changed significantly with advances in digital technologies. Spurred by a new Batman trilogy starring Christian Bale, released between 2005 and 2012, and the inception of the so-called Marvel Cinematic Universe with 2008's Iron Man, it was now common for multiple superhero movies to number amongst the top blockbusters each year.

New York City was the home of various Marvel Comics superheroes, and the inspiration for Metropolis and Gotham City

Moffat himself enjoyed superheroes in both their comic book and screen incarnations. Having written romantic sitcoms like Coupling, he appreciated the unusual dynamics they offered, such as Superman being the object of reporter Lois Lane's affections, while Lois was pursued by Clark Kent -- Superman's secret identity. Moffat thought that a Doctor Who story should play with many of these conventions. As such, the setting would be New York City; the home of various Marvel Comics superheroes like Spider-Man and the Fantastic Four, it was also the inspiration for Metropolis in the adventures of Superman and for Batman's native Gotham City. Moffat also sought opportunities to incorporate other popular elements of superhero lore, such as Superman's ability to disguise himself as Clark Kent simply by donning a pair of glasses. A suitably larger-than-life group of villains was drawn from The Husbands Of River Song, which had included representatives of the Shoal of the Winter Harmony in a supporting role.

Moffat was writing the Christmas special during July, with a first draft of the script ready in mid-August. At this stage, the gemstone which gave Grant his powers was known as the Last Tears of God's Eye. It later became Hazandra, the Ghost of Love and Wishes, which helped to explain Grant's choice of superheroic moniker. Journalist Lucy Fletcher was devised as an analogue of Lois Lane. Indeed, since Lucy's married surname was Lombard, the two characters shared the same initials; Lucy was also the name traditionally given to Lois' younger sister, while Lombard was the surname of one of Lois' colleagues. The globe atop the Harmony Shoal building was an allusion to a similar architectural feature which adorned the offices of the Daily Planet, the Metropolis newspaper which employed both Clark Kent and Lois Lane. The Ghost's costume was envisaged as having elements of both Superman's and Batman's attire.

Recording for Season Thirty-Six had begun in late June, with Matt Lucas rejoining the cast as Nardole, the character he had played in The Husbands Of River Song. Moffat had expected Lucas to be available for only a handful of episodes, but it quickly became clear that the actor was willing to commit a significant amount of time to Doctor Who. Consequently, by early August, Moffat decided to reintroduce Nardole in the 2016 Christmas special, rather than in the subsequent season premiere, The Pilot. The script pages he had already written were revised accordingly.

When Doctor Who was first transmitted in Mexico during 1968, it was given the title Doctor Misterio

Around the same time, Moffat recalled a conversation he had had with Peter Capaldi regarding the August 2014 Doctor Who world tour, which had promoted the broadcast of the Twelfth Doctor's first season. The penultimate stop on the tour had been Mexico City, where Capaldi was delighted to learn that, when Doctor Who had first been transmitted in the country during 1968, it was given the title Doctor Misterio -- that is, “Doctor Mystery”. The actor was enchanted by the translation, and Moffat felt that it sounded like a superhero's name, in the manner of Marvel Comics' Doctor Strange or DC Comics' Doctor Fate. As such, he decided to call the Christmas special The Return Of Doctor Mysterio.

Ed Bazalgette was engaged to direct the adventure, which would be made as the year's third production block. He had previously handled Season Thirty-Five's The Girl Who Died and The Woman Who Lived. In the interim, he had also directed three episodes of the spin-off Class, including Capaldi's appearance in the series premiere, For Tonight We Might Die. The Return Of Doctor Mysterio would be his final contribution to Doctor Who. Bazalgette felt that Moffat's script evoked the spirit of Reeve's Superman films, as well as 2002's Spider-Man starring Tobey Maguire. He viewed the relationship between Grant and Lucy as being akin to Reeve's Superman and Margo Kidder's Lois Lane, or Maguire's Spider-Man and Kirsten Dunst's Mary Jane Watson. Cast as Grant, Justin Chatwin also looked to Reeve's portrayal of Superman, while his use of a deeper voice for the Ghost was suggested by Michael Keaton's Batman in the 1989 movie.

Doctor Who's usual production home of Roath Lock Studios in Cardiff was the primary venue for the first three days of recording on The Return Of Doctor Mysterio, from September 5th to 7th. The first day was devoted to the brain vault at Harmony Shoal. Work on this material continued to the 6th, when the shot of Brock leaving the ambulance was also taped. Bazalgette then started filming TARDIS sequences, which he completed on the 7th. Cast and crew concluded the day at the Royal Welsh College of Music & Drama in Cardiff. It offered a space suitable as the main area of the Harmony Shoal buildings -- including the London offices, which were omitted in editing -- and production continued there through September 9th.

Following the weekend, Bazalgette's team returned to the College on the 12th and 13th. Part of the latter day was spent back in the studio, for an insert of the Ghost smashing the Harmony Shoal office window, and then the start of recording in Lucy's apartment. Virtually all of the remaining production schedule for The Return Of Doctor Mysterio was confined to Roath Lock, including more material in the flat from September 14th to 16th.

The apartment sequences were completed after the weekend, on September 19th and 20th. Shots on the fire escape were also taped on the first day, while Bazalgette's itinerary on the second day included inserts of the Ghost outside the Harmony Shoal window, the start of material aboard the spaceship, and the scene atop the Empire State Building. The latter set was built horizontally; this made it easier to capture shots of the young Grant as he levitated for the first time, since it meant that juvenile actor Logan Hoffman could lie on his back. This technique was inspired by the Sixties Batman series, which often depicted the Caped Crusader ascending the outside of a building in a similar manner. The spaceship set was the focus on September 21st and 22nd. The latter day also saw a start on the rooftop sequences involving the young Grant. Bazalgette completed this material on the 23rd, alongside the Doctor's conversation with the teenaged Grant in the high school corridor.

Another break over the weekend preceded a week which was chiefly devoted to scenes on the roof of Lucy's apartment building. The only other shooting on September 26th was aboard the now-wrecked spaceship. Material in the young Grant's bedroom was taped on the 27th and 28th; the former also saw the Doctor and the boy ascend the staircase, which was actually part of the backlot for the Welsh-language soap opera Pobol y Cwm. Much of September 29th was spent on effects sequences, including both the young and adult versions of Grant in flight and the Doctor swinging outside the young Grant's window. The only day during the week which did not involve any filming on Lucy's rooftop was the 30th. Instead, Bazalgette completed the scenes in the young Grant's bedroom, the report about the children in peril at the amusement park, news footage of the Ghost, and a pick-up shot of Grant holding the spaceship aloft.

Two days were spent in Sofia, Bulgaria, where a sophisticated Manhattan backlot had been erected

At an early stage, it had been hoped that the Doctor Who budget might allow elements of The Return of Doctor Mysterio to be filmed on location in New York City itself. Instead, it was agreed that the production would spend two days at Nu Boyana Studios in Sofia, Bulgaria, where a sophisticated Manhattan backlot had been erected in 2007. A minimal crew -- of which the only notable castmembers were Chatwin and Charity Wakefield, who was playing Lucy -- flew to Sofia on Sunday, October 2nd, with recording following on the 3rd and 4th. A movie theatre was seen to be showing a film called Mind Of Evil, in reference to the 1971 serial The Mind Of Evil. Finally, some inserts were taped at Roath Lock on November 16th.

Additional dialogue added in post-production included Brock's reference to Miss Shuster and Miss Siegel; they were named in homage to the creators of Superman. Another comic book allusion could be found in the advance publicity for The Return Of Doctor Mysterio. Here, Grant's surname was given as Gordon; this echoed the frequently alliterative secret identities given to Marvel Comics superheroes, such as Peter Parker (Spider-Man) and Bruce Banner (the Hulk).

On November 18th, the public was given the opportunity to sample nearly three minutes of The Return Of Doctor Mysterio, when the Harmony Shoal office sequence which built to the first appearance of the Ghost aired as part of the Children In Need charity appeal. The entire special then aired on Christmas Day as usual. Like The Husbands Of River Song, The Return Of Doctor Mysterio was also made available in cinemas, with fans in Australia, Canada, Denmark and New Zealand able to watch the theatrical presentation on Boxing Day.

• Doctor Who: The Complete History #85, 2018, “Story 264: The Return Of Doctor Mysterio”, edited by John Ainsworth, Hachette Partworks Ltd.