The Doctor finds himself transported to a mysterious castle, where the walls revolve like gears. Outside is only the sea, its bed laden with ancient skulls. Inside, the rooms seem haunted by nightmares drawn from the Doctor's own memories. Stalking the halls is a shrouded creature; its pursuit of the Doctor is virtually ceaseless, pausing only when the Time Lord reveals one of his secret truths. As he explores the castle, the Doctor uncovers clues which point towards a particular room hidden deep within the edifice. But he also begins to suspect that he is embroiled in a trap which was sprung a very long time ago...

From its earliest days, it was acknowledged that the format of Doctor Who effectively discouraged any consideration of the lasting consequences of the Doctor's actions. Some stories had tried to buck this trend: the last two episodes of 1966's The Ark showed the Doctor revisiting the setting of the first two installments and dealing with the legacy of his earlier involvement, while 1977's The Face Of Evil saw the Doctor dealing with a menace he had inadvertently created during an unseen prior escapade. But even these adventures tended to skirt around the emotional fallout of the Doctor's travels; when companion Adric died in 1982's Earthshock, it was all but forgotten by the end of the next serial.

During Doctor Who's 2011 season, a key event was the kidnap of Melody Pond, the newborn daughter of companions Amy Pond and Rory Williams. The storyline invited some criticism because of the limited personal impact demonstrated by the characters; instead, the nature of the programme essentially required Amy and Rory to move on to their next adventure. Executive producer Steven Moffat became determined to provide space for a more sophisticated exploration of grief in Doctor Who, and a prime opportunity arose four years later.

Having Clara's death earlier in the season would leave time to explore the Doctor's reactions

Season Thirty-Five would be the last for Jenna Coleman as companion Clara Oswald, and Moffat had decided that the character would be killed when her attempts to imitate the Doctor finally went too far. Although he originally intended for these events to take place in the finale, by February 2015 he had realised that Clara's death would be more effective if it occurred earlier in the season. Not only would the development come as a greater surprise to the audience, but it would also leave him time to explore the Doctor's reactions to the loss of his best friend.

As such, Clara would now meet her demise in the season's tenth episode, Face The Raven. Episode eleven would then see the Doctor dealing with his grief, which Moffat felt would require a script that focussed solely on the Time Lord, to the exclusion of any other characters. To this end, he sought permission from Charlotte Moore, BBC One's Controller of Programmes, and Polly Hill, the BBC's Controller of Drama Commissioning, to write a one-hander, in which Peter Capaldi would carry the story solo. This would allow Moffat to investigate how the Doctor behaved when he was on his own, and the extent to which his demeanour around his companions was a guise he adopted for their benefit.

Moffat completed the first draft of his script in early June. A crucial element of the adventure -- ultimately entitled Heaven Sent -- was a transmat device which could create additional versions of the Doctor. This notion had first occurred to Moffat during the Eighties, and he had considered using it as the basis of a script for Big Finish Productions when the company launched its series of original Doctor Who audio dramas in 1999. He now decided that the Doctor would be caught in a loop which would see him die and be recreated billions of times; however, most of the events depicted in the episode would be drawn from just one iteration, partway through the sequence. The story's castle setting would not completely reset each time, and so the Doctor would come across evidence of some of his past selves' activities, such as the ageing portrait of Clara or the missing flagstone which was later found buried in the garden. In early versions, one of these clues led the Doctor to acquire an alarm clock in a castle workshop, which he used to measure the intervals before the Veil caught up with him.

Heaven Sent was originally to be made as part of Season Thirty-Five's sixth production block alongside the season finale, Hell Bent. In the event, however, the latter was delayed to Block Eight and so Heaven Sent was recorded on its own. The director was Rachel Talalay, who had impressed Moffat with her work on the previous year's climactic adventure, Dark Water / Death In Heaven. For visual inspiration, she looked to the German Expressionist film movement, which had produced works like Das Cabinet des Dr Caligari (The Cabinet Of Dr Caligari, 1920) and Nosferatu: Eine Symphonie des Grauens (Nosferatu: A Symphony Of Horror, 1922) and often made use of weird geometries and unnatural lighting. Aside from Capaldi and Coleman, the only credited performer in the episode was Jami Reid-Quarrell, playing the Veil; Reid-Quarrell had recently portrayed Colony Sarff in the season premiere, The Magician's Apprentice / The Witch's Familiar.

Rachel Talalay recorded slightly different versions of many shots to depict later iterations of the cycle

The majority of Heaven Sent was recorded at Doctor Who's regular production facility, Roath Lock Studios in Cardiff, starting with TARDIS scenes on June 24th and 25th. Part of the latter day was also devoted to the green screen effects of the Doctor floating in the sea. The 26th saw work begin on the material in the garden and the adjacent hallway, prior to a break for the weekend. In accordance with the script, Talalay recorded several slightly different versions of many shots for use in the montage which depicted later iterations of the cycle; Moffat wanted to convey the fact that the Doctor's experiences were not identical each time through the loop.

The rest of the garden footage was captured on June 29th; across the two days of shooting on the set, the pit was dug progressively deeper, ultimately extending six feet downwards. The first location day for Heaven Sent was June 30th, which took cast and crew to Cardiff Castle. In addition to various corridors and stairs, it offered spaces suitable for the ballroom, the library, the smoking room and the cog room. July 1st was spent at Caerphilly Castle in Caerphilly. There, Talalay recorded sequences at the sea door and in the adjacent chamber where the Doctor changed into dry clothes, on the battlements from which he observed the sea, and in the dining room. More action in the hallways and on the staircases was filmed, as was an insert in the garden. An aerial drone was used to capture the Doctor's point of view as he dove towards the water.

Back in the studio, July 2nd concentrated on scenes in the bedroom where the Doctor found Clara's portrait. Elements of the Doctor's fall were also recorded, although the majority of this sequence was completed the next day using a special rig which would allow Capaldi to perform while lying on his back, instead of the usual practice of being suspended from wires. Shots atop the tower were also captured on the 3rd, alongside various inserts and footage of the flies, which had been deemed too numerous to be accomplished via computer animation. The skull recovered from the sea was actually based upon a cast of Capaldi's head. Production then stood down for two weeks, during which the Doctor Who team would be engaged in various promotional exercises, including an appearance at the San Diego Comic-Con.

As a result, the last full week of filming for Heaven Sent didn't begin until July 20th, with Talalay's team reassembling at Roath Lock. First on the itinerary was the material in the pantry, before attention shifted to Room 12 and the adjacent corridor. Room 12 and the azbantium tunnel remained the focus on the 21st and 22nd. Part of the latter day also saw footage recorded in the teleport chamber and the hallway outside. Cameras continued to roll in the teleport chamber and various corridors on July 23rd. A late addition to the script at this stage was the Doctor's closing comment about the Hybrid, alluding to the mystery which had been referenced throughout the season, about a legendary threat to Gallifrey. The 24th was principally devoted to the Doctor's initial encounter with the Veil, plus numerous pick-up shots.

Coleman's contributions were taped on the TARDIS set on August 10th, alongside further inserts. The closing scene on Gallifrey was filmed during work on Hell Bent in Spain's Canary Islands. The locale was the desert outside Corralejo on Fuerteventura, where the footage was recorded on August 27th; this work likely continued to the 28th. On September 8th, shots of the submerged Doctor were captured in a water tank at Black Hangar Studios in Lasham, Hampshire. The final pick-ups required for Heaven Sent were then completed at Roath Lock on September 15th.

Although Doctor Who had been regularly granted a fifty-minute timeslot during Season Thirty-Five, it was agreed that the nature of Heaven Sent precluded significant edits, and a fifty-five-minute duration was authorised. Nonetheless, some trims were necessary. Notably, the Doctor's opening description of the Veil replaced his discovery of a similar message on the wall of the circular corridor, while some of the rooms he was meant to discover in the castle -- such as the ballroom -- were not shown. Heaven Sent was broadcast at 8.05pm on November 28th, starting five minutes earlier than Face The Raven the week before. Coleman's name was omitted from the opening credits to hide her involvement with the episode.

• Doctor Who: The Complete History #83, 2018, “Story 261: Heaven Sent”, edited by John Ainsworth, Hachette Partworks Ltd.