The TARDIS brings the Doctor and Rose to Sanctuary Base Six, a research facility on a planet which -- against all the laws of physics -- orbits a black hole. Meeting the crew, the Doctor is astonished to find that they have unearthed writing so ancient that even the TARDIS can't translate it. The goal of the expedition is to drill miles below the planet's surface and locate the incredible power source which is counteracting the black hole's massive gravitational pull. But Toby Zed, the team archaeologist, is being haunted by a malevolent voice. The telepathic Ood servants are behaving very strangely. And the Beast has awakened...

Although Doctor Who's return to television in 2005 was widely hailed by audiences and reviewers alike, it was not immune to criticism. One of the most frequent objections levelled at Season Twenty-Seven was the absence of alien environments, with all thirteen episodes taking place either on Earth or aboard a space station in Earth orbit. This was an intentional decision by executive producer Russell T Davies; in addition to taking a cautious approach towards the programme's budget, he felt that every Doctor Who story needed a human connection to seem relevant to the viewer, and he worried that audiences would find the depiction of other planets to be implausible. For Season Twenty-Eight, however, Davies was confident enough to attempt an adventure that would be set, at least in part, on another world.

The story idea which Davies developed for this experiment involved explorers on a planet called Hell who became possessed by demonic forces. It drew upon the 1987 John Carpenter film Prince Of Darkness, in which physics students investigated an ancient cylinder containing the embodiment of Satan in liquid form, who then took control of them one by one. Davies wanted to present the Doctor with a challenge to his own belief systems, and confront him and Rose with a scenario in which their trust in each other would provide the resolution, as opposed to the Doctor's prior knowledge and experiences. Davies was also responding to producer Phil Collinson's observation that none of the Doctor's enemies during the 2005 season had operated on a nigh-omnipotent scale, as had been the case for memorable twentieth-century villains such as the Osirian god Sutekh (1975's Pyramids Of Mars) or the ancient evil Fenric (1989's The Curse Of Fenric).

Matt Jones obtained permission to use the Slitheen, now enslaved by humanity

The writer assigned to this narrative was Matt Jones, who had written the 1996 Doctor Who: The New Adventures novel Bad Therapy for Virgin Publishing. Jones was working on his scripts from February 2005, initially under the umbrella title “Satan Pit”. A key change involved Toby's continued possession in Episode Two. Originally, Jones intended to hold this revelation back until the serial's climax, but Davies felt that doing so seemed too abrupt. Jones also felt that his early drafts lacked the diversity which would be provided by a tangible alien presence. He obtained Davies' permission to use the Slitheen, first introduced in 2005's Aliens Of London / World War Three. Now enslaved by humanity, the Slitheen secretly believed that the planet orbiting the black hole was actually the dwelling place of their god, who would free them from servitude.

Of great concern was the form in which the Beast would manifest. Although “Satan Pit” was intended to air midway through the season, it would be made towards the end of the schedule, when money was typically starting to run low. Davies worried that the production would not be able to afford an elaborate computer-animated creation, and that a prosthetics approach would not be sufficiently credible. Instead, he suggested that the Beast might appear as an old man or a gigantic disembodied eye; another option was for the entity to take the form of a young girl, inspired by one of the avatars of the Senior Partners in the vampire drama Angel. Davies also briefly considered tweaking the storyline to replace the Beast with the corrupted future humans he had originally offered Robert Shearman for use in 2005's Dalek, and which would ultimately become the Toclafane of The Sound Of Drums / Last Of The Time Lords in 2007. In the end, however, a compromise was reached on other digital elements which would enable the Beast to be realised through computer animation after all.

The two episodes of “Satan Pit” were accorded the individual titles The Impossible Planet and The Satan Pit. They were scheduled to be made as the season's fourth recording block, under director Euros Lyn. At a late stage, however, Davies became concerned that the presence of the returning Slitheen was detracting from the humans whose pioneer spirit he felt was at the heart of the tale. He discussed the matter with Millennium Effects, who provided many of the practical elements for Doctor Who. They confirmed that they could create a small set of simple alien masks within the budget originally allocated to refurbish the Slitheen costumes. On this basis, Davies devised a new alien race which he dubbed the Ood. Their simple name was a deliberate contrast to his typical flair for ostentatious alien nomenclature, and reflected his intention that they seem “odd”. Their uniform appearance was based upon the eponymous aliens of 1964's The Sensorites, with Davies imagining that the two races hailed from sister planets. The Ood were given long facial fronds and a translation device to avoid the need for an expensive mouthpiece or complex animatronics.

Filming for The Impossible Planet / The Satan Pit would take place in parallel with Love & Monsters

With additional time now required to amend Jones' script to replace the Slitheen with the Ood, the production schedule was rearranged. The Idiot's Lantern and Fear Her were assigned to Block Four, with The Impossible Planet / The Satan Pit instead comprising Block Five. As a result, the serial would be directed by James Strong, who was new to Doctor Who but was familiar to both Davies and executive producer Julie Gardner. To save time and reduce the strain on David Tennant and Billie Piper, Block Five would be made in parallel with the year's sixth and final recording block, which included only Love & Monsters. A similar double-banking schedule had previously been employed during Season Twenty-Seven, when The Long Game had entered production while filming was still under way for The Empty Child / The Doctor Dances.

An early notion for The Impossible Planet / The Satan Pit was to confine its recording almost entirely to the studio. The production team resisted the idea of using a quarry to film material on the alien planet, fearing that they would be perpetuating a hoary Doctor Who cliche. By the start of 2006, however, it had been agreed that such an environment would be the best way to achieve the scale that Jones' story demanded. As a result, cameras rolled at Wenvoe Quarry in Wenvoe from February 28th to March 3rd, capturing scenes in the cavern which contained the seal on the Beast's prison. Since an open quarry was standing in for an enclosed cave, extra dialogue was written to cover the possibility of rain or snow, with the Doctor explaining either one as poisonous precipitation produced by gases circulating beneath the planet's surface. Part of March 3rd was also spent at HTV Wales Studios in Cardiff, recording footage of the Doctor's descent into the pit.

Four days followed at the regular Doctor Who studio space of Unit Q2 in Newport, spanning March 6th to 9th, for scenes on the Sanctuary Base walkways and access tunnels. On the 10th, Tennant travelled with the crew to Clearwell Caves, at Clearwell in Gloucestershire, where he filmed the material in the Beast's prison. March 11th was again spent at Wenvoe Quarry, which now stood in for the planet's surface as well as its interior. Scenes in the rocket were filmed on March 13th and 14th at Enfys Television Studios in Cardiff; part of the first day was also devoted to material in Toby's room. Cast and crew shifted back to Unit Q2 on the 15th and 16th, for recording on the habitation area set. Strong's team then returned to Enfys on March 17th, where he concentrated on segments involving the capsule and the pit interior.

The Johnsey Estates factory in Pontypool played host to The Impossible Planet / The Satan Pit for six days, from March 20th to 25th. It offered spaces suitable for both the Ood holding pen and the bore room, and replaced a different venue which had been found to be severely contaminated with asbestos. Next on the agenda was a week at Unit Q2, from March 27th to 31st. The set for the command centre was Strong's primary focus, alongside various inserts. Part of the 30th was also spent recording action on the walkways, while the final day saw the completion of the TARDIS scenes. With recording on Love & Monsters having started and finished while The Impossible Planet / The Satan Pit was in production, March 31st marked Billie Piper's final day of filming as a series regular.

Both of writer Gareth Roberts' TARDISodes for The Impossible Planet / The Satan Pit were recorded on April 1st. The prelude to the first installment was taped at BBC Broadcasting House in Cardiff. It featured Captain Walker -- who had died by the start of the serial itself -- being tasked with the expedition to the black hole. The fifty-eight-second item ended with Walker being watched by an Ood, who uttered a prophecy of the Beast. Roberts originally imagined these events being set in a docking station, rather than an executive office.

The TARDISode for The Satan Pit was filmed on the command centre set at Unit Q2. This was the final Doctor Who-related recording at the facility; a new purpose-built dedicated studio space had been constructed for the show at Upper Boat near Pontypridd, and the only standing set -- the TARDIS -- would be relocated there during the spring. Roberts' script served as a sequel of sorts to its predecessor. It depicted a Sanctuary Base crewmember discovering documents relating to the legend of the Beast amongst the personal effects of the now-deceased Walker, only to fall victim to the Beast's supernatural influence. Running to fifty-seven seconds, the item concluded with another crewmember finding the man's corpse, covered in symbols. The TARDISode was intended to tie into references in The Impossible Planet to people having previously died in the command centre, but the relevant dialogue was excised in editing.

On April 1st, a water tank at Pinewood Studios was used for the shots of Scooti and Toby floating through space

Work on The Impossible Planet / The Satan Pit also continued on April 1st, with a water tank at Pinewood Studios in Iver Heath, Buckinghamshire used for the shots of Scooti and Toby floating through space. On April 4th, the smoke and flame elements needed for various special effects shots were recorded at Ealing Studios in London. Formerly the BBC Television Film Studios, these facilities had regularly hosted Doctor Who during its original run, but had been sold in the mid-Nineties. Finally, a pick-up shot of Rose's hand unlatching Toby's belt buckle was taped at Enfys on April 11th.

Effects house The Mill offered its entire staff the opportunity to design the Beast, and the successful submission came from Grant Bonser, who was working in the company's dispatch department. Bonser drew upon the Lord of Darkness, as played by Tim Curry in the 1985 fantasy film Legend, balanced against the production team's desire for a primordial creature which did not lean too heavily on common cliches about the Devil's appearance. Although the Beast was depicted as towering over the Doctor, 3D artist Nicolas Hernandez's final version was nonetheless much smaller than Davies had initially envisaged. In fact, he had imagined a creature so large that the Doctor would mistake the Beast's eyelid for a part of the cavern wall. For the voice of the Beast, Davies and Collinson agreed that they wanted an actor with a vocal presence akin to Gabriel Woolf, who had provided the dialogue for Sutekh in Pyramids Of Mars. As it turned out, Woolf was happy to return to Doctor Who -- indeed, he had recently appeared in two Sixth Doctor audio dramas for Big Finish Productions -- and he duly recorded his part on April 27th.

The TARDISode for The Impossible Planet was released on May 27th; the episode itself aired the following Saturday, June 3rd, which was also the premiere date for The Satan Pit's TARDISode. This was the last week that Strictly Dance Fever led into Doctor Who, as the reality competition reached its season finale. The broadcast of The Satan Pit on June 10th was instead preceded by a celebrity edition of the quiz show Weakest Link.

• Doctor Who Magazine Special Edition #14, 9th November 2006, “Episodes 8 & 9: The Impossible Planet / The Satan Pit” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #52, 2016, “Story 174: The Impossible Planet / The Satan Pit”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Inside Story by Gary Russell (2006), BBC Books.