When the TARDIS is mysteriously dragged off-course to Bristol, Clara goes investigating while the Doctor tries to figure out what's wrong with the Ship. But when something in the vicinity begins leeching its external dimensions, the Doctor finds himself trapped within a rapidly shrinking time machine. The mystery deepens as Clara learns about a series of disappearances which have plagued the area in recent weeks. With the help of a young graffiti artist named Rigsy, she finds herself thrust into the Doctor's role, battling invading forces which quite literally hail from another dimension.

A longtime fan of Doctor Who, Jamie Mathieson had discussions about writing for the show in both 2004 and 2010, but without success. By 2013, however, Mathieson's work had caught the attention of Sue Vertue, a television producer who also happened to be the wife of Doctor Who executive producer Steven Moffat. Vertue encouraged her husband to give Mathieson another opportunity to pitch ideas for his programme. Aware that his earlier suggestions had dwelt too much on high concepts at the expense of offering engaging adventures for a general audience, Mathieson developed four concepts to present to Moffat. He discussed them with well-known Doctor Who enthusiast Toby Hadoke -- whom Mathieson knew from his days as a stand-up comedian -- and was reassured that the series had not previously explored similar ground.

This time, Mathieson's meeting with Moffat bore fruit, with the writer asked to develop a proposal involving two-dimensional monsters which were invading the three-dimensional universe. A key image was the 1533 painting The Ambassadors by Hans Holbein the Younger. Although ostensibly a portrait of diplomats Jean de Dinteville and Georges de Selve, the artwork was best-known for the presence of a large skull, rendered in anamorphic perspective such that it could be clearly interpreted only by an observer standing at an oblique angle to the canvas.

Jamie Mathieson thought in terms of cartoons such as 1949's Fast And Furry-ous with Wile E Coyote and the Road Runner

Several other sources also inspired Mathieson. Edwin A Abbott's 1884 novella Flatland: A Romance Of Many Dimensions told of a Square inhabiting the two-dimensional Flatland, who encountered various inhabitants of worlds with differing numbers of dimensions. Flat Stanley was a 1964 children's novel by Jeff Brown, in which a boy was flattened beneath a fallen bulletin board, only to discover the unexpected possibilities which his two-dimensional existence now offered. The fourth of PJ Hammond's surreal Sapphire & Steel serials, broadcast in 1981, involved an entity which could trap people in photographs. Finally, Mathieson thought in terms of various Looney Tunes cartoons, in which a recurring gag involved a character's ability to enter a painting which others could not. For instance, 1949's Fast And Furry-ous saw Wile E Coyote paint a false tunnel mouth on a cliff face, only to watch in bewilderment as his quarry, the Road Runner, ran into it and back out again.

Mathieson's monsters became known as the Boneless, which he imagined sliding over people, crushing their bones and allowing their bodies to be piloted. Although Moffat was uncertain about the practicalities of realising the Boneless on-screen, Mathieson nonetheless proceeded to a draft script under the title Flatline, which would be positioned as the ninth episode of Season Thirty-Four. At this stage, a key group of characters was a gang of young homeless graffiti artists; one of them, Luc, would be the first victim of the Boneless. The story then followed Rigsy Moorhouse, who was named for the best man at Mathieson's wedding. He and the survivors of a Community Payback crew would discover the Doctor and Clara in the service tunnels, already in the process of investigating the Boneless after meeting the terrified homeless teenagers.

The train driver -- called Perkins at this stage, after another friend of Mathieson's who was a locomotive aficionado -- entered the proceedings at an early stage, accompanying a group of refugees from a train car which had been attacked by the Boneless. The monsters continued to pick off the humans, at one point even setting up an ambush by piloting the bodies of several police officers. Fenton died saving one of the teenagers, who was known as Hashtag. The Doctor was finally able to locate the vessel which had transported the Boneless from their homeworld into three-dimensional space. Rigsy painted a false door in front of the ship, and when the monsters tried to un-flatten it, they triggered their vessel's recall protocol, dragging them back to their own realm. Flatline ended with a coda set several years in the future in which Hashtag, now grown to adulthood, finished telling the story of the Boneless as a warning to others.

In some drafts, the Doctor sought help from Kate Stewart and UNIT

In subsequent drafts, the first victim of the Boneless became an aging hippie called Morton. The production team wanted Mathieson to depict Peter Capaldi's newly-minted Doctor discovering the mysterious goings-on, so the TARDIS now arrived in Bristol at the start of the story. A new complication was the proximity of a busy train platform, which led the Doctor to seek help from Kate Stewart and the Unified Intelligence Taskforce (UNIT), revisiting the character from 2012's The Power Of Three and 2013's The Day Of The Doctor. The epilogue with the adult Hashtag was dropped.

In early October, however, Mathieson was forced to make major changes to Flatline when he was informed that it was to be double-banked with another script. The other story would concentrate on the Doctor, while Mathieson's adventure would principally focus on Clara; as such, scenes requiring Capaldi would have to be kept to a bare minimum. This prompted Mathieson to expand upon an element of his earlier drafts, in which the size of the TARDIS was affected by the Boneless. Originally, the Doctor had been able to reverse the phenomenon, but he would now become trapped inside the shrinking time machine. The notion of the Boneless inadvertently activating their recall protocol was replaced by the restoration of the TARDIS. The homeless teenagers were excised at this stage, while Morton evolved into Roscoe.

By now, the production team had become very impressed by Mathieson's ingenuity -- such as the joke based on The Addams Family, a Sixties sitcom and Nineties movie franchise derived from the macabre work of cartoonist Charles Addams. The writer soon found himself offered the opportunity to develop the story which go into production at the same time as Flatline. This would become Mummy On The Orient Express, and it would occupy much of Mathieson's attention over the subsequent months.

Nonetheless, Flatline continued to be refined during the early part of 2014. The train passengers and Kate Stewart were omitted; with the importance of the train driver now reduced, Mathieson decided to use the surname Perkins for a key character in Mummy On The Orient Express instead. The driver in Flatline was instead called Bill, which had previously been the name of one of the passengers. Briefly, he became an old friend of Fenton's and explained that the other man's bitterness was because he had once been an anti-authoritarian himself. Only at a late stage was it decided that Fenton would survive the adventure after all.

Flatline was designated Block Five of the Season Thirty-Four production calendar. It would be directed by Douglas Mackinnon, who had recently completed Block Two: Listen and Time Heist. Work began on May 28th, by which time Mummy On The Orient Express had already been filming for a week. First, shots of Danny in the park were captured at Britannia Quay in Cardiff Bay. Cast and crew then shifted to the nearby Roath Lock Studios, where Capaldi recorded some of his solo material in the TARDIS console room. On the 29th, Roscoe's residence was actually the day room of Channel View Flats in Cardiff. Sequences in the derelict railway tunnel were taped on May 30th at the Barry Shooting Range in Barry, after which Saturday the 31st was a day off.

Production on Flatline resumed in Barry on June 1st, with the train storage shed actually located at the Cambrian Transport depot. On the 2nd, Mackinnon's team travelled to Winchcombe, Gloucestershire, where a privately-owned rail line provided the venue for the adventure's climactic scenes, plus the shot of the passing train seen in the opening minutes. No filming took place on June 3rd, because Jenna Coleman was attending the Glamour Women of the Year Awards ceremony, where she was named Best UK TV Actress. Material in the underpass was taped on the 4th and 5th at the Maelfa Shopping Centre in Cardiff; part of the latter day was also spent back at the Cambrian Transport depot, this time for events in the maintenance room. Capaldi then rejoined the Flatline team on June 6th to tape more TARDIS sequences at Roath Lock.

Full-body scans would be married to motion-capture performances to create the Boneless

Cast and crew enjoyed a long weekend before production resumed on the standing TARDIS set on June 10th. The same day, a number of full-body scans were performed at the premises of Ten24 in Sheffield, South Yorkshire; they would be married to motion-capture performances in order to create the Boneless. Work on Block Four had now concluded, making Capaldi available for the rest of the Flatline shoot. He recorded additional TARDIS sequences at Roath Lock on the 11th, after which Mackinnon's team returned to the Cambrian Transport depot for shots aboard the train, plus the remaining material in the storage shed. It was back to Roath Lock on June 12th for the action in Mr Heath's home, as well as several inserts. Barry was once again the filming location on the 13th, when scenes in the wasteland where the TARDIS materialised were shot at the Barry Tourist Railway, while the exterior of Mr Heath's home was actually the Owl Manor Bed and Breakfast.

Following the weekend, more elements of the climax in the railway tunnel were taped at Roath Lock on June 16th, alongside additional pick-up shots. The last of the TARDIS scenes were filmed at Roath Lock on the 18th, as was the Doctor's emergence from the restored time machine and various effects shots. Later the same day, a new version of Danny's phone call was taped at Enfys Television Studios in Cardiff, to account for changes which had latterly been made to Clara's dialogue. The closing shot of Missy -- a belated addition to the script -- was also recorded there. Finally, more inserts were captured at Roath Lock on July 4th, and at the Mamhilad Park Industrial Estate in Pontypool on July 21st.

A scene dropped from Flatline in editing saw the Doctor boost Clara's cell phone signal so that the survivors of the Community Payback crew could call their loved ones. Rigsy declared that he had no one to call, while Fenton admitted that the person he wanted to phone would hang up on him. As a result, Rigsy offered to make the call for him; the subsequent dialogue implied that he spoke to the mother of Fenton's daughter. When Flatline aired on October 18th, it was scheduled at the slightly earlier time of 8.25pm.

• The Doctor Who Companion -- The Twelfth Doctor: Volume Three, February 2020, “Flatline” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #79, 2018, “Story 250: Flatline”, edited by John Ainsworth, Hachette Partworks Ltd.