When Danny is killed in a traffic accident, Clara demands that the Doctor save him. The Doctor decides to take Clara beyond the mortal veil, using the TARDIS telepathic circuits to plot a course. They materialise in the halls of an organisation called 3W, where the corridors are lined with skeletons floating in a mysterious dark water. There they meet the enigmatic Missy, and learn that Danny now dwells in a realm called the Nethersphere. But when Missy reveals herself to be a very old acquaintance of the Doctor's, the time travellers realise that they have fallen into a trap which transcends life and death.

The Master had not been seen in Doctor Who since the concluding episode of The End Of Time, broadcast on New Year's Day 2010. It was generally assumed that this would be the last appearance for the incarnation which John Simm had played since 2007's Utopia, with the actor leaving the programme alongside the Tenth Doctor, David Tennant, and executive producers Russell T Davies and Julie Gardner. Davies' successor, Steven Moffat, had decided to rest the character during Matt Smith's era as the Eleventh Doctor, but the allure of the Time Lord villain was too strong to resist as he began planning the Twelfth Doctor's debut season. To revitalise the character -- and wrong-foot the viewing audience -- the new Master would be female, originally nicknamed “Misty” and later “Missy”.

Moffat planned to seed Missy into various episodes throughout Season Thirty-Four, building to the revelation of her true identity in the two-part season finale. This was a format that he had not employed since The Pandorica Opens / The Big Bang at the end of Season Thirty-One, but he felt it was now time to return to an epic-length capstone to the year. Missy's appearances would be accompanied by allusions to the afterlife, initially referred to as the Promised Land and then the Nethersphere. Moffat thought that an exploration of what might happen after death was fertile new ground for Doctor Who, providing a source of dread to which adult viewers would be more susceptible.

As a child, the Cybermen had been Steven Moffat's favourite monsters

For the younger members of the audience, Moffat decided to bring back the Cybermen. They had been his favourite monsters as a child but, to date, he had only been able to write for them as a small component of broader stories like The Pandorica Opens and Season Thirty-Two's A Good Man Goes To War. Although Neil Gaiman had recently relaunched the Cybermen in Nightmare In Silver during Season Thirty-Three, Moffat decided that the Cybermen of the finale would be a new breed created by Missy from the Earth's dead.

Moffat began working on the two-part story in earnest around the late winter of 2014. One of the obstacles that he faced was that he had little conception of how to characterise Missy, beyond a vague thought that she would be a young woman with an insane smile. One of the few firm decisions he had made was that she would be revealed as the woman responsible for bringing the Doctor and Clara together in Season Thirty-Three's The Bells Of Saint John, having previously decided against resolving the mystery in that year's concluding story, The Name Of The Doctor.

Everything changed when Michelle Gomez was suggested as a suitable candidate to play Ms Delphox in Season Thirty-Four's fifth episode, Time Heist. Scheduling conflicts forced Gomez to ruefully decline the role, but Moffat had already realised that the actress was the ideal candidate to play Missy. He immediately began to develop the character with Gomez in mind, balancing a sense of turbulent menace against a vein of biting humour, and he was delighted when the actress accepted the part. Gomez decided against familiarising herself with the Master's previous incarnations, although she recalled Roger Delgado's performance as the original Master during the early Seventies.

The production team wanted a director with a strong background in science-fiction for the season finale. They turned to Rachel Talalay, whose career dated back to the sixth movie in the A Nightmare On Elm Street franchise, released in 1991. Talalay would be the first American-born director to work on Doctor Who, although her mother hailed from England; she had become a fan of the programme while spending time in the United Kingdom during her teenaged years. Now residing in British Columbia, Canada, Talalay travelled to Wales in early May. At this stage, Moffat was only able to furnish her with a draft of the opening installment, which had become known as Dark Water. Nonetheless, most of the key action was already in place, including the emergence of the Cybermen onto the steps of St Paul's Cathedral, which intentionally evoked the well-remembered cliffhanger of 1968's The Invasion Episode Six.

Jenna Coleman's contract only ran through the end of Season 34

For the concluding installment, entitled Death In Heaven, Moffat was aware that there was a possibility that this might be Clara Oswald's final Doctor Who episode. Actress Jenna Coleman's contract only ran through the end of Season Thirty-Four, and a decision had not yet been made as to whether she would remain with the show. As such, Moffat structured the narrative to provide a potential close to Clara's arc. At this stage, the Doctor did not convince her to hand over Missy's weapon; instead, it was Clara who appeared to vaporise the evil Time Lord. Moffat also indicated that the Doctor's features in the opening title sequence should be replaced with with Clara's, as an extra hook for casual viewers.

Moffat wanted Death In Heaven to establish the scale of Missy's villainy by having her murder a significant character. He selected Osgood, a scientist with the Unified Intelligence Taskforce (UNIT) who had become a fan favourite following her introduction in the previous year's fiftieth-anniversary special, The Day Of The Doctor. Moffat was already planning a story for Season Thirty-Five which would include the death -- kept intentionally ambiguous -- of either Osgood or her Zygon doppelganger. He now decided to bring this event forward, and have Missy be responsible for the murder. To add to the shock of this development, he wrote Osgood's early scenes as if the way was being paved for her to become the Doctor's next companion.

The first draft of Death In Heaven was completed towards the end of May; at this stage, the little boy whom Danny had inadvertently killed during his time as a soldier had lines of dialogue throughout the episode. Minor changes were made to both scripts during the first two weeks of June, including Brigadier Lethbridge-Stewart taking Clara's place as Missy's apparent executioner. Perhaps most significant was the addition of an extra scene at the end of Death In Heaven. Whereas it had originally concluded with Clara watching the TARDIS dematerialise -- seemingly for the last time -- there was now a teaser for the 2014 holiday special, Last Christmas, with Santa Claus entering the TARDIS.

Dark Water / Death In Heaven was made as the sixth production block of Season Thirty-Four; episode ten, In The Forest Of The Night, was held back to form Block Seven since it would benefit from the resulting midsummer recording dates. The first week of work on the finale largely took place at Enfys Television Film Studios in Cardiff. From June 16th to the morning of the 18th, Talalay's focus was the material on the Nethersphere balcony. The latter part of the 18th through to the 20th saw her attention shift to scenes in Clara's flat. An insert on the balcony was also taped on the 19th, the same day that the Trade Street Café in Cardiff served as the venue for the Doctor and Clara's goodbye. The final hours of work on the 20th saw cast and crew return to their usual production home at Roath Lock Studios in Cardiff. Its costume preparation area again posed as the Nethersphere meeting room, as had been the case for episode six, The Caretaker.

Following the weekend, recording for Dark Water / Death In Heaven resumed on June 23rd at the National Museum of Wales in Cardiff, which offered the marble staircase at the entrance to the 3W vault. The day ended back at Roath Lock, where cameras would be rolling for the rest of the week. The remainder of the 23rd and the first part of the 24th were spent on the volcano set, which was repurposed from the children's series Wizards Vs Aliens. Talalay then filmed sequences in the 3W vault through to June 27th. Part of the latter day was devoted to the sequence of Clara entering the TARDIS from her apartment, prior to another weekend off.

Although cast and crew would be spending two days in London towards the end of the shoot, there wouldn't be enough time to capture all of the necessary footage outside St Paul's Cathedral. As a result, much of this material was actually performed at the Friary in Cardiff on June 30th. Gomez's involvement with Doctor Who was announced by the BBC the same day, although her character was described only as “the Gatekeeper of the Nethersphere”. From July 1st to 4th, Box Cemetery in Llanelli served as the graveyard where the climactic events of Death In Heaven took place. Part of the latter day was also spent in a hangar at MOD St Athan, where a real aircraft undergoing maintenance at the facility posed as the exterior of Boat One. Meanwhile, a second unit recorded various inserts at the former St Illtyd's Boys' College in Cardiff, alongside some ultimately-unused shots of a Cyberman at the gates of a school.

The production again stood down on Saturday and Sunday, prior to a week confined to Roath Lock. Talalay's major concern was action aboard Boat One, starting in the cabin on July 7th and 8th. The set included a portrait of the late Nicholas Courtney as Brigadier Lethbridge-Stewart, based upon a publicity shot from 1989's Battlefield. Talalay shot the scene of a Cyberman pulling Colonel Ahmed through the window as an homage to Freddie Krueger's attack on Marge Thompson at the end of 1984's original A Nightmare On Elm Street. Green screen shots of Kate falling from Boat One were also taped on the 8th, and the day ended on the set for the hold, which was a redressed version of the space shuttle from the season's seventh episode, Kill The Moon. This work continued to the 11th, and included Gomez ad-libbing Missy's rendition of Mickey, a 1982 hit for Toni Basil which was itself adapted from Racey's 1979 song Kitty. Some of the scenes in Dr Chang's office were also recorded on the 11th, as was Missy's descent by umbrella to the graveyard. This green screen effect was inspired by the 1964 movie Mary Poppins, with Moffat envisaging Missy as a sinister version of the magical nanny played by Julie Andrews.

The setting for Danny's fatal accident was City Hall Road in Cardiff

Talalay's team remained at Upper Boat after the weekend, from July 14th to 16th. The material in Dr Chang's office was completed on the first two days, followed by action on the standing TARDIS set during the last two days. Shots of the Doctor and Kate falling through the air were also captured against a blue screen on the 14th. Cast and crew were back out on location in Cardiff on July 17th, starting along Queen Street for pick-up shots of Clara bidding farewell outside the TARDIS. The day's main venue was the area around Alexandra Gardens: the exterior of the Chaplet Funeral Parlour was actually the Glamorgan Building, while the setting for Danny's fatal accident was around the corner, on City Hall Road.

July 18th and 19th saw production shift to London. On the first day, more graveyard shots were captured at the expansive Brompton Cemetery in West Brompton, including some -- ultimately dropped in editing -- that were intended to represent events at other burial sites around the world. The second day was spent in the City of London, at St Paul's Cathedral and along St Peter's Hill. Talalay's elder daughter, Sophie, played one of the plainclothed UNIT operatives, while her younger daughter, Lucy, was amongst the civilians. To prevent press leaks about Missy's true identity, one take had Gomez identify herself as a “Random Access Neural Interface” or RANI, suggesting that she might be a new incarnation of the evil Time Lord played by Kate O'Mara in three stories starting with 1985's The Mark Of The Rani. For the version actually used in the finished programme, Gomez and Capaldi mouthed their dialogue, which would be added in dubbing.

After a day off on Sunday the 20th, the last day of principal photography on Dark Water / Death In Heaven was July 21st, when work took place at the Mamhilad Park Industrial Estate in Pontypool. The flashbacks to Danny's military service were recorded there, alongside the material in the funeral parlour and various inserts. Talalay then completed more pick-up shots at Roath Lock on July 30th. By now, Coleman had decided that she enjoyed working with Capaldi too much to bow out of Doctor Who quite yet. It was agreed that Clara would return in Last Christmas, with Santa's dialogue in the Death In Heaven coda implying that she and the Doctor still had unfinished business. This scene was filmed at Roath Lock during the production of the festive special; most of the work took place on September 15th, followed by some inserts on the 22nd.

Doctor Who continued to creep earlier in the Saturday schedule during the broadcast of Dark Water / Death In Heaven. On November 1st, the opening installment aired at 8.15pm, while the season finale on November 8th was pulled back to 8.00pm. Death In Heaven was accorded a one-hour timeslot, and although Strictly Come Dancing still led into Doctor Who, Casualty was rested this week. Instead, the finale was followed by The National Lottery Live and then the 2014 edition of the Royal British Legion Festival Of Remembrance. With Season Thirty-Four having come to an end, the second season of Atlantis took Doctor Who's place in the BBC One schedule.

• The Doctor Who Companion -- The Twelfth Doctor: Volume Three, February 2020, “Dark Water / Death In Heaven” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #79, 2018, “Story 252: Dark Water / Death In Heaven”, edited by John Ainsworth, Hachette Partworks Ltd.