A distress call brings the Doctor, Bill and Nardole to the Chasm Forge space station in the far future. There they find that only four out of forty crewmembers are still alive -- stalked by their deceased colleagues, whose corpses are still propelled by their exoskeletal Smartsuits. Someone or something has reprogrammed the suits, issuing new instructions to murder the station's crew. To make matters worse, this is an era in which oxygen is a highly regulated commodity. To continue breathing, the time travellers must risk everything and don the lethal Smartsuits themselves.

After completing The Girl Who Died for Season Thirty-Five, Jamie Mathieson embarked upon a new Doctor Who script. However, executive producer Steven Moffat soon deemed the concept too dark, and the story was abandoned. Instead, Moffat asked Mathieson to develop an adventure which would represent new companion Bill Potts' first experience in space. Inspired by movies like 2013's Gravity, in which two astronauts tried to survive after their shuttle was destroyed during a spacewalk, Moffat wanted to explore the extent to which the vacuum of space was inherently inimical to human life.

Mathieson conceived a new storyline entitled Oxygen. It involved crystalline space parasites, whose queen could mimic the sound of a distress signal in order to draw spaceships to their doom. The parasites could leap from vessel to vessel, and their gaze allowed them to influence people's thoughts. When the Doctor and Bill arrived, several ships -- including some that were at war with each other -- had been lured to the parasites' nest. As the parasites picked off the ship's crews one by one, the survivors were forced to make difficult decisions due to their limited reserves of oxygen. Another source of drama was the paranoia which stemmed from the uncertainty over which individuals had been brainwashed by the parasites. There would also be a climactic conflict about whether the parasite queen should be allowed to survive.

At one stage, the zombies were being created by a crystalline entity living in an asteroid

By late November 2015, Moffat had asked Mathieson to develop a more detailed outline for Oxygen. Considering the original brief, Mathieson decided to reemphasise the theme of space as the enemy. Instead of the mind-controlling parasites, the ships would now be plagued by their own dead, who had turned into zombies after being exposed to the absence of air pressure in the vacuum. Forming an uneasy alliance with a group of survivors led by Kline, the Doctor and Bill found a variety of ways to navigate between the different vessels. The Doctor eventually discovered that the zombies were being created by a crystalline entity living in a nearby asteroid, which he convinced to cease its attack.

Mathieson wrote the initial draft of his Oxygen script during March and April 2016. The zombies were known as Carriers, since they could now infect others with a touch. Having been peppered with questions about Doctor Who by twelve-year-old Ivan, the son of his friend Darren, he used both names for members of the spaceship crew; the latter became Dahh-Ren, who was originally described as having an arachnid face. Kline had now become Tasker, and there were two other survivors: Maddox, a female soldier, and a male hacker called Karlo. With the Doctor and Bill, they made their way through various spaceships while evading the Carriers. Aboard one, the Doctor reversed the gravity so that they could pull themselves along furniture bolted to the floor while the Carriers were trapped on the ceiling.

Intent on keeping their numbers small in order to conserve oxygen, Tasker murdered Karlo because his skills were deemed redundant in comparison with the Doctor's. He also tried to kill Bill, before the Doctor observed that he actually offered less to the group than any of the others. Tasker later sacrificed himself to slow down the Carriers. Maddox became a Carrier, and she was able to act as a conduit for the gestalt entity of which the Carriers were a part. She revealed that the infection started when a mining ship began excavating strange crystals from a nearby asteroid. Maddox infected Ivan, who killed Dahh-Ren. The Doctor was able to reactivate one of the vessels, using its missiles to destroy the asteroid and the ships, while teleporting himself and Bill back to the TARDIS.

Although Moffat liked Mathieson's script, he felt that its ambition needed to be scaled down. For his part, Mathieson thought that the elements of the Carriers and the search for oxygen tended to be in competition, rather than complementing each other. Instead of being animated by the effects of the crystals, Moffat suggested that the Carriers could instead be corpses propelled by their spacesuits. Mathieson agreed with this approach, although he was aware that the monsters would have to be presented differently from the Vashta Nerada of 2008's Silence In The Library / Forest Of The Dead, which had taken the form of spacesuited skeletons.

Ganymede Systems was named in allusion to Gus, the villain of Mummy On The Orient Express

Mathieson now decided that the murderous spacesuits and the limited oxygen would both be consequences of decisions made by the unscrupulous Ganymede Systems corporation. He chose the name as an allusion to Gus, the artificial intelligence which had been the villain of his Season Thirty-Four story, Mummy On The Orient Express. Mathieson now intended Oxygen to serve as a prequel to the earlier adventure, explaining why Gus had been so determined to snare the Doctor.

The second draft of Oxygen, finished by the end of May, was virtually a complete rewrite. Instead of a spaceship graveyard, the setting was now the Chasm Forge space station. It was named in reference to the Valley Forge, a space freighter in the 1972 film Silent Running, which itself referred to the site of an encampment for George Washington's Continental Army during the American Revolution. Maddox was dropped, while Tasker was now a woman. It was Tasker who survived electrocution to find herself amongst the spacesuit corpses; however, she subsequently suffocated because she was unable to obtain more oxygen. Mathieson reused the name Kline for a Ganymede Systems bureaucrat who confronted the Doctor at the story's climax. After his threat to blow up Chasm Forge provided him with the opportunity to get the survivors aboard the TARDIS, the Doctor destroyed the space station anyway. In order to provide another example of the dangers of living in outer space, this draft added the idea of the Doctor being blinded as a result of his exposure to the vacuum, with his eyesight beginning to return at the end of the adventure.

Oxygen continued to evolve during the summer. It was now known that Matt Lucas' availability for Doctor Who was more substantial than originally assumed, so Nardole could be added to the script. This, in turn, offered an opportunity for Bill to replace Tasker as the person who survived electrocution. Karlo became Abby, Tasker's gender reverted to male, Kline was omitted, and the spacesuits were branded Smartsuits. Most significantly, Moffat agreed to Mathieson's suggestion that the Doctor's blindness should not be cured at the end of the episode. The executive producer now wanted to explore the ramifications of his sacrifice in subsequent stories.

By the end of August, the script had been renamed “Zero Hour”. This was because there was concern that the subplot about oxygen conservation wasn't substantial enough to warrant the original title. However, it was then decided that Mathieson had achieved a suitable balance between this element and the killer Smartsuits, and the adventure became Oxygen again by mid-September. Dahh-Ren was now a blue-skinned alien, to spare the expense of fabricating a spider-like prosthetic.

A fortnight elapsed between the end of Block 3 and the start of Block 4, accommodating a promotional junket

Oxygen was paired with The Eaters Of Light -- then planned to be episode nine, but ultimately episode ten -- as Block Four of the Season Thirty-Six production calendar. Returning to direct the stories after a ten-year absence from Doctor Who was Charles Palmer, whose most recent work on the programme had been 2007's Human Nature / The Family Of Blood. A fortnight elapsed between the end of Block Three -- The Return Of Doctor Mysterio -- and the start of Block Four, accommodating a promotional junket in Canada and a panel at the New York Comic Con.

As a result, recording got underway on October 17th, the first of four consecutive days at Roath Lock Studios in Cardiff. Palmer captured action in the airlock where the TARDIS materialised and the adjacent corridor, before moving on to the workshop. The latter set was in use again on the 18th and 19th -- when the airlock was also modified to become the hub -- with some corridor sequences taped on the latter day as well. In his blue make-up as Dahh-Ren, actor Peter Caulfield soon found himself nicknamed “Blue Peter”, in reference to the long-running children's programme. More corridor material was filmed on October 20th, when the cameras were also rolling on the standing TARDIS set. The last day of work for the week was the 21st, which saw Palmer's team make a start on scenes in Chasm Forge's processing area at Aberthaw Power Station in West Aberthaw.

Recording at Aberthaw Power Station concluded after the weekend on October 24th, with the venue now providing a space suitable as the mech bay where Ivan inspected Bill's Smartsuit, alongside further shots in the processing area. Back at Roath Lock, footage of the long corridor outside the processing area was also captured. Both the airlock and long corridor sets were in use on the 25th. The premises of G24 Power, a solar cell manufacturer in Newport, served as Chasm Forge's power core on October 26th and 27th. Part of the latter day was also spent in the studio, where the vault chamber set used earlier in the season had been re-erected. The Chasm Forge hull was the main focus during work at Roath Lock on the 28th, together with some additional filming in the long corridor.

Cast and crew returned to Roath Lock following the weekend, with sequences in the construction zone recorded on October 31st and November 1st. Also on Palmer's itinerary for the latter day was the closing scene in the Doctor's office at St Luke's University, plus several inserts. On November 2nd, the Doctor's lecture was held at the Reardon Smith Lecture Theatre, part of the Nation Museum of Wales in Cardiff. Assistant director Lauren Pate played the student who asked about crop rotation. Various pick-up shots for Oxygen were taped at Roath Lock on November 18th. Studio filming on the 22nd focussed on wirework, for both the Smartsuit corpses floating in space and the TARDIS crew in the workshop during decompression. Palmer also took the opportunity to remount the workshop material in which the Doctor, Bill and Nardole first encountered a Smartsuit corpse, since the original make-up on extra Tim Reid had now been deemed too gruesome.

On November 30th, the opening sequence involving Ivan and Ellie was taped at Roath Lock. The Empire State Building mast built for The Return Of Doctor Mysterio was repurposed for these shots, and additional wirework was also undertaken. Then, in early 2017, Moffat decided to give the vault subplot a greater presence throughout the first half of Season Thirty-Six. With new scenes added to episodes three and four -- Thin Ice and Knock Knock -- the vault chamber footage recorded for Oxygen in late October was no longer entirely suitable. As such, Palmer filmed additional sequences on the vault chamber set at Roath Lock on February 22nd. He also captured a closing shot which revealed the vault's occupant to be Missy, the Doctor's Time Lord arch-nemesis. It was taped later the same day, at the Coal Exchange in Cardiff, but was dropped in editing.

Oxygen aired on May 13th. It started at 7.15pm, five minutes earlier than the preceding episodes of Season Thirty-Six, because it would be followed by the 2017 edition of the Eurovision Song Contest.

• Doctor Who: The Complete History #87, 2018, “Story 269: Oxygen”, edited by John Ainsworth, Hachette Partworks Ltd.