Clara agrees to accompany the Doctor on one final trip in the TARDIS, so he takes her aboard the luxurious Orient Express -- or rather, its recreation as a hyperspatial passenger train. However, they soon learn that the Express is haunted by a legendary mummy called the Foretold. Unseen by anyone else, it appears to its victim precisely sixty-six seconds before it takes their life. Soon, strange coincidences start to pile up -- such as the fact that one of the passengers just happens to be an expert in alien mythology. Even as the body count rises, the Doctor begins to realise that somebody is playing a very dangerous game...

By late October 2013, the early drafts of Jamie Mathieson's Flatline had earned a positive reception from the Doctor Who production team. Intended to be the ninth episode of Season Thirty-Four, Flatline was Mathieson's first work for the series, but he was nonetheless asked to immediately embark upon a second script. Executive producer Steven Moffat recalled that Season Thirty-One's The Big Bang had ended with the Doctor being summoned to deal with “an Egyptian goddess loose on the Orient Express, in space”. He hadn't intended it to be anything more than a one-line gag, but it now inspired the title which he recommended to Mathieson: Mummy On The Orient Express.

As Mathieson was well aware, the underlying source for this title was the classic Agatha Christie mystery novel Murder On The Orient Express. Originally published in 1934, it followed detective Hercule Poirot as he solved the killing of businessman Samuel Ratchett aboard the titular passenger train. A cross-continental service operated by the Compagnie Internationale des Wagons-Lits from 1883, the Orient Express most commonly travelled between Paris, France and Istanbul, Turkiye.

The monstrous mummy was a chiefly cinematic tradition which had its heyday during the Thirties

The monstrous mummy was a chiefly cinematic tradition which had its heyday during the Thirties. In 1932, Universal Pictures seized upon the widespread interest in Egyptology which had been inspired by the opening of King Tutankhamun's tomb a decade earlier. The result was The Mummy -- with Boris Karloff as the undead high priest Imhotep -- and its subsequent imitators. Hammer Films launched a series of mummy movies in 1959 with The Mummy starring Peter Cushing and Christopher Lee. More recently, Universal reimagined the concept in an action-orientated vein for 1999's The Mummy with Brendan Fraser. For its part, Doctor Who had seized upon mummy lore only once before, as a disguise for the servitor robots of 1975's Pyramids Of Mars.

Although Mathieson was happy to tackle a second Doctor Who assignment, he was wary of the constraints that a mummy narrative would impose. Mummies were often depicted as slow-moving and bound to their sarcophagi, which would be particularly problematic in a story confined to a train. Indeed, he felt that these limitations were part of the reason that mummies had enjoyed only occasional bouts of popularity, compared to evergreen movie monsters like vampires and werewolves. To overcome his concerns, Mathieson conceived the Foretold as a type of mummy which could be perceived only by its victims, and its sarcophagus as something other than what it appeared. He initially imagined that the Foretold's bandages would be able to operate independently, but this element was eventually dropped.

Mathieson was also informed that Mummy On The Orient Express would have to include only a minimal role for Clara, much as he had been required to limit the Doctor's involvement in Flatline. This was because the two stories would be double-banked -- the practice of having multiple episodes in production simultaneously -- which had been a routine consideration since 2005 in order to complete an entire regular season as well as a Christmas special within the allotted recording schedule. In this case, Jenna Coleman would spend most of her time making Flatline, which would form the season's fifth production block, while Peter Capaldi would principally work on Mummy On The Orient Express. The latter would be paired with episode seven, Kill The Moon, as Block Four.

In the original version of Mummy On The Orient Express, the space train was touring the Seven Wonders of the Universe, starting with the ruined Fortress of Riskar. This element was inspired by the Egyptian cruise at the heart of another Christie mystery, 1937's Death On The Nile. Perkins, the chief engineer, was named for a friend of Mathieson's who was a locomotive aficionado. The first victim was called Mrs Fanshaw; at this stage, she was unrelated to Maisie Pitt, who was a professor of archaeology. The train's captain was the odious Mr Carnival; Quell was the name of a man who claimed to be a military general, but was in fact Erasmus Such, the train's owner, in disguise.

The Foretold suits could originally manipulate the power of probability

It transpired that Such was dying of a wasting disease, and he had opened a portal to the long-lost city of Mere on Riskar in order to summon the Foretold. Teleported to Mere, Clara and Maisie discovered that the Foretold were actually Riskarians who had become dependent on a protective suit which could prolong life indefinitely; Such aimed to acquire this technology. The suits could manipulate the power of probability and, in fact, all of the Foretold's victims aboard the Express had been killed due to the advent of extremely unlikely circumstances. With more and more Foretold coming through the portal from Mere, the passengers reprogrammed the navigation computer to take the Express to the haunted planet Cromlaw. The Doctor then teleported the passengers to the TARDIS, while the dark matter monsters which infested Cromlaw attacked the train, causing it to explode.

By late February 2014, Mathieson had dropped the Forsaken's ability to affect chance, since he thought it was too reminiscent of the improbable deaths which were the hallmark of the 2000 horror movie Final Destination and its sequels. It was also agreed that the dark matter monsters detracted from the Forsaken, so the Cromlaw element was abandoned. Voices of the Damned -- actually the Riskarians trapped inside their survival suits for millennia -- were now heard pleading for help. The Doctor used his sonic screwdriver to hack the suits, allowing the Damned to finally die.

During March, Moffat worked with Mathieson to simplify the plot of Mummy On The Orient Express. Riskar and the other Wonders of the Universe were omitted, and there was now just a single Foretold which was a soldier left over from a long-forgotten war. Quell and Carnival were combined into a single character, while Erasmus Such was replaced by the more oblique villainy of the computer, Gus. Mrs Fanshaw became Maisie's mother, and then her grandmother. Previously, Mathieson's premise had been that the Doctor and Clara were on holiday, inspiring a running gag that involved both time travellers investigating the mysterious goings-on while trying to keep their activities secret from each other. However, Moffat had now positioned Mummy On The Orient Express as the season's eighth episode, in the aftermath of Clara's furious row with the Doctor at the end of Kill The Moon. Consequently, the trip aboard the Express would be presented as Clara's farewell excursion.

As part of Block Four, Mummy On The Orient Express would be directed by Paul Wilmshurst. He was well-versed in Egyptology, having lived in Cairo, Egypt for a time as a child. Wilmshurst had previously worked with the story's principal guest star: comedian Frank Skinner, who would portray Perkins. Skinner was a long-time Doctor Who fan who had already been seen in the affectionate spoof The Five(ish) Doctors Reboot for the show's fiftieth anniversary in 2013. Meanwhile, Mrs Pitt would be played by Janet Henfrey, who had previously had a role in 1989's The Curse Of Fenric. Amongst other actresses considered for the part was Laura June Hudson, a former Doctor Who costume designer who had begun acting in recent years.

Foxes would sing Freddie Mercury's Don't Stop Me Now, originally recorded by Queen in 1978

The most unusual addition to the supporting cast for Mummy On The Orient Express had actually been arranged several weeks before any of the other guest artistes. This was singer-songwriter Foxes, for whom the script was amended to replace a jazz trio with a vocalist. Born Louisa Rose Allen, Foxes had contributed to Zedd's 2012 Grammy Award-winning dance track Clarity. She had spoken enthusiastically about Doctor Who after meeting Moffat at a BBC Worldwide Showcase event in Liverpool on February 24th, prompting the invitation to appear in the programme shortly afterwards. As the start of production approached, it was decided that Foxes would sing Freddie Mercury's Don't Stop Me Now, originally recorded by Queen in 1978. Doctor Who's regular composer, Murray Gold, would prepare a new jazz-infused arrangement for the episode. Other late changes to the script resulted from Moffat's suggestion that the sixty-six-second countdown should appear on screen. Consequently, Mathieson amended these sequences to ensure that the timing was authentic.

Although Mummy On The Orient Express was made almost entirely at Roath Lock Studios in Cardiff, work began at the Power House on May 20th. Located within the Cardiff Bay Business Centre, it was the site for scenes in Danny Pink's apartment. The 22nd and 23rd were spent in the studio, working in the laboratory car. On the first day at Roath Lock, the costume for the Foretold -- designed by Millennium FX and worn by Jamie Hill -- made its debut. Millennium had originally taken an approach orientated towards science-fiction elements, only to learn that the production team was keen for the Foretold to draw heavily upon the classic image of a scary, bandaged cadaver.

A weekend off preceded the only exterior filming for Mummy On The Orient Express, with Clara awakening on Aberthaw Beach at Limpert Bay in West Aberthaw on May 26th. It was back to Roath Lock on the 27th, for scenes in the lounge car, the storage car, and corridors. May 28th was the only day that Foxes was available, since she was in the midst of a tour to promote her album Glorious. In addition to the lounge car material for which she was required, Wilmshurst also taped action in the Doctor and Clara's cabins, the baggage area, the storage car, various corridors, and at the door to Carriage 24. Work on May 29th began in the Doctor's room, and continued to the corridors before returning to the laboratory; recording on the latter set continued to the 30th.

Following the weekend, June 2nd saw filming resume in the laboratory and at the door to Carriage 24. On the 3rd, Wilmshurst taped scenes in Quell's office and Perkins' workroom alongside several inserts. June 4th concentrated on the lounge car, including its transformation into the laboratory. Production on the 5th began with an ultimately-deleted shot of the deserted TARDIS, in which Clara left a voicemail message to alert the Doctor that she was trapped in Carriage 24. The rest of the day was devoted to green screen effects, notably for the kitchen staff being ejected into space. Having spent the preceding week making Flatline, Coleman rejoined the Mummy On The Orient Express team on June 6th to start recording the sequences involving Clara and Maisie in Carriage 24.

The production again stood down for the weekend before the action in Carriage 24 was completed on June 9th. Wilmshurst then taped Perkins' scene aboard the TARDIS -- during which Skinner asked if there was time to film an alternative take, in which the engineer agreed to join the Doctor in his travels -- as well as a number of pick-up shots. Principal photography for Mummy On The Orient Express concluded on June 10th, which began with Mrs Pitt's death in the dining car, prior to the chef's demise in the kitchen.

Mummy On The Orient Express aired on October 11th. The entire Saturday evening schedule was slightly delayed compared to recent weeks, and so Doctor Who did not begin until 8.35pm. This would be the latest start time ever for a regular first-run episode of the programme; only the seven-minute Children In Need Special (2005) had been positioned farther into the night. On this occasion, The National Lottery Live was brought forward to air in-between Mummy On The Orient Express and that week's installment of Casualty.

• The Doctor Who Companion -- The Twelfth Doctor: Volume Two, October 2019, “Mummy On The Orient Express” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #78, 2017, “Story 249: Mummy On The Orient Express”, edited by Mark Wright, Hachette Partworks Ltd.