Gallifrey has been returned to the universe, and the Doctor has been returned to Gallifrey. His homecoming is anything but triumphant, however, as he finds himself hunted by Lord President Rassilon and the High Council. They demand that the Doctor divulge his knowledge of the Hybrid, the legendary entity that will stand in the ruins of Gallifrey. But he is determined to right the wrong of Clara's death, even if it means breaking the laws of time to rescue her from her final moments. The Doctor's grief takes him to the very end of the universe, where he must face the truth about the Hybrid... and make a terrible choice.

By the middle of 2015, it was clear that the final episode of Doctor Who's thirty-fifth season would have a number of tasks to accomplish. From an early stage, executive producer Steven Moffat had planned for it to be set on Gallifrey, resolving the Doctor's search for his home planet which had begun in 2013's The Day Of The Doctor. The Doctor would be pursuing a selfish agenda arising from one of the year's key events: the death of companion Clara Oswald in episode ten, Face The Raven. This development, too, would be revisited, providing a more optimistic -- if bittersweet -- resolution to the character's lengthy tenure. Another presence would be Ashildr, the immortal Viking introduced in episode five, The Girl Who Died, and developed over the course of episode six, The Woman Who Lived, and then Face The Raven.

Tying all of these threads together would be the Gallifreyan legend of the Hybrid, a being said to emanate from two warrior races. It was first mentioned in episode two, The Witch's Familiar, when Davros, the creator of the Daleks, referred to his belief that the Hybrid was half-Dalek and half-Time Lord. Further allusions to the Hybrid were then made throughout the season. In The Girl Who Died, the Doctor described Ashildr as a hybrid after she gained immortality by absorbing a Mire battlefield medical kit. In episode seven, The Zygon Invasion, the Doctor referred to his friend Osgood as a hybrid because it was unclear if she was the human original or her Zygon duplicate. Most explicitly, episode eleven, Heaven Sent, would see the Doctor trapped in a Time Lord prison for billions of years because he refused to divulge what he knew about the Hybrid. The season finale would pick up in the immediate aftermath of these events, with the Doctor arrival on Gallifrey after escaping the prison.

Steven Moffat wanted to revisit some established elements of Gallifreyan mythos while introducing new ideas

When Doctor Who returned to television in 2005, the destruction of Gallifrey in the Time War had been established in order to free the programme from the burden of its extensive mythos. Even its subsequent involvement in 2009's The End Of Time and its restoration in The Day Of The Doctor had permitted only a limited exploration of the Time Lord planet, since the setting was subordinate to a larger narrative. Moffat wanted to buck this trend for the season finale, revisiting some established elements -- such as the tyrannical presidency of Rassilon, last played by Timothy Dalton in The End Of Time -- while introducing new ideas.

One of Moffat's goals was to make better use of the notion of regeneration; apart from the Doctor, only his companion Romana and the villainous Master had actually been seen to undergo the process. This prompted the notion of the Doctor dealing a mortal wound to another Time Lord because he knew that the other individual still had regenerations remaining. It was also an opportunity to reinforce the concept that a Time Lord's incarnations were not restricted to a particular gender. This idea had already been established during Season Thirty-Four with the introduction of Missy, a female regeneration of the Master, a character who had previously been seen only in male incarnations.

Moffat's initial draft was completed towards the start of July; the adventure would ultimately be called Hell Bent. An inspiration for the early part of the story was Shane, Jack Shaefer's 1949 Western novel about an enigmatic stranger who refused to carry a gun, yet was roused to the defence of a group of homesteaders who were being intimidated by a cattle driver and his gang. Moffat indicated that the clothes donned by the Doctor after arriving at the barn -- a location first seen in The Day Of The Doctor and again in Season Thirty-Four's Listen -- should emulate the attire worn by Alan Ladd when he starred in the celebrated 1953 movie adaptation of Shane.

There were only minor changes to Hell Bent during the development of the script. Originally, Ohila's involvement in the narrative ended with the Doctor instigating her installation as the new President of the High Council. Dialogue between the Doctor and Ashildr referenced his surprise that the Mire battlefield medical kit had not yet ceased to function. Ashildr then implied that she had somehow found a way around this technological limitation.

An early plan for the making of Season Thirty-Five was that Hell Bent would be paired with the year's penultimate episode, Heaven Sent, as the sixth block of the recording schedule. In the event, however, the four episodes which formed Blocks Five and Six were split into individual productions; Hell Bent would be the last of them, designated Block Eight. Nonetheless, it would continue to share director Rachel Talalay with Heaven Sent, which remained as Block Six.

Maisie Williams was now filming Game Of Thrones, so her availability for Hell Bent would be severely limited

In casting Hell Bent, Talalay hoped that Dalton would be able to reprise the role of Rassilon. However, she soon learned that he was committed to the dark fantasy series Penny Dreadful during the required filming dates. Instead, veteran actor Donald Sumpter would play a new incarnation of the legendary Time Lord. His extensive resume included two previous Doctor Who appearances -- in 1968's The Wheel In Space and 1972's The Sea Devils -- as well as The Eternity Trap, a 2009 serial which aired as part of the spin-off series The Sarah Jane Adventures. Meanwhile, Maisie Williams returned for a fourth and final appearance as Ashildr. Her involvement came as something of a surprise to the actress, who had assumed that Face The Raven marked the end of her character's story. Williams was now involved with the early stages of filming for the sixth season of the fantasy drama Game Of Thrones, and so her availability would be severely limited.

Production on Hell Bent began with a week at Roath Lock Studios in Cardiff from August 10th to 14th. The main focus was the set for the Cloisters, which was adapted from elements constructed for the castle in Heaven Sent. The Cloister Wraiths glided using concealed Segways in the same manner as Colony Sarff in the season premiere, The Magician's Apprentice / The Witch's Familiar. As such, Sarff actor Jami Reid-Quarrell was hired not only to play one of the Sliders, but also to coach his fellow artistes. The Dalek encountered by the Doctor and Clara was one of the casings originally constructed for 2005's Bad Wolf / The Parting Of The Ways, and so it was already exhibiting considerable wear and tear.

In addition to the Cloisters, a portion of August 13th and 14th was devoted to scenes in the extraction chamber. In this material, the General became the fourth Time Lord to regenerate on-screen. Ken Bones had played the character in The Day Of The Doctor -- and had provided the voice emanating from Gallifrey in 2013's The Time Of The Doctor -- and he now gave way to T'Nia Miller, who had been in Cucumber and Banana from former Doctor Who executive producer Russell T Davies. The 14th also saw the completion of material aboard Gastron's sky tank, on the Capitol rooftop, in the lift to the Cloisters, and on a section of the trap street set retained from Face The Raven.

With Doctor Who at the mercy of the Game Of Thrones production schedule, Talalay found her plans for the second week of recording on Hell Bent upended when Williams abruptly became unavailable. Work resumed after the weekend with scenes in the American restaurant on August 17th. The venue was Eddie's Diner on Cardiff Bay; as implied in the Doctor's dialogue, this was the same location visited for 2011's The Impossible Astronaut. The 18th was spent on the set for the old-style TARDIS console room, which had originally been constructed for the 2013 docudrama An Adventure In Space And Time. It was relocated to Roath Lock from the nearby Doctor Who Experience, where it normally formed part of the standing exhibition. A lifelong Doctor Who fan, Peter Capaldi reviewed clips of the First Doctor, William Hartnell, so that he could adopt a similar approach to his use of the TARDIS controls. Unfortunately, it was discovered that the interior doors had been built too short, meaning that Talalay had to exercise caution when shooting the characters entering or leaving the set.

Cameras were rolling on both the classic and modern TARDIS console rooms on August 19th, while the scene of the Doctor and Clara running along a hallway to the lift was also shot at Roath Lock. Cast and crew then relocated to a unit on the Dominion Way Industrial Estate in Cardiff, where the barn set originally created for The Day Of The Doctor had been erected; it, too, normally formed part of the display at the Doctor Who Experience. Work at the industrial park continued to the 20th. It was back to Roath Lock on August 21st and 22nd, for the remaining sequences aboard the old-style TARDIS; the High Council chambers were also in use on the first day. Williams was able to rejoin the Doctor Who team on the 22nd, which was also Coleman's last day on the show. A door was borrowed from Eddie's Diner for the sequence in which Clara bade farewell to the Doctor.

On August 25th, Doctor Who returned to the Canary Islands for the third time during the making of Season 35

After a day off on Sunday the 23rd, scenes in the High Council chambers were completed on August 24th. The 25th was a travel day, as Doctor Who returned to Spain's Canary Islands, off the coast of Morocco, for the third time during the making of Season Thirty-Five. As with The Zygon Invasion, the easterly island of Fuerteventura would host Talalay's team. Filming took place in two stretches of desert near Corralejo, first to represent the surface of Gallifrey on August 26th and 27th, and then to stand in for the Nevada dunes on the 28th. Unfortunately winds were high during the shoot, which resulted in a number of sandstorms. Talalay had to abandon some shots, while there was also difficulty controlling the aerial drone which captured Gastron's perspective from the sky tank. Cast and crew then returned to the United Kingdom on August 29th.

Although principal photography on Hell Bent had now concluded, two significant elements remained to be shot. As such, an additional day at Roath Lock was scheduled for September 15th. Williams was able to be present as Talalay taped all of the sequences in the ruins of the Cloisters at the end of time. Next, the shot of the Doctor catching his new sonic screwdriver was taped on the standing TARDIS set; it had been deferred to provide sufficient time for the prop to be completed. Moffat had decided to introduce a remodelled sonic screwdriver because he felt that the previous version was too closely identified with Matt Smith's Eleventh Doctor. Various inserts were also recorded in the studio on the same day.

Hell Bent was allocated a sixty-five minute timeslot for its broadcast on December 5th. It was transmitted from 8.00pm, five minutes earlier than Heaven Sent the week before. The next regular occupant of Doctor Who's place in the BBC One schedule would be the fifth season of the singing competition The Voice UK. However, Doctor Who itself would return in less than three weeks with its annual Christmas special. The gap which separated Hell Bent and The Husbands Of River Song represented the shortest amount of time between the regular season and the holiday broadcast since the tradition began in 2005.

• Doctor Who: The Complete History #84, 2018, “Story 262: Hell Bent”, edited by Mark Wright, Hachette Partworks Ltd.