The Doctor and Martha find themselves trapped on board a spaceship spiralling into a sun. To complicate matters further, one of the crew has seemingly become possessed and has started murdering his compatriots, having somehow gained the ability to incinerate at a glance. The time travellers have just 42 minutes to avert disaster or they, and everyone on board the vessel, will burn alive.

Chris Chibnall had been a key figure in the development of Torchwood, taking on the role of head writer and penning four episodes for its first season. In July 2006, halfway through Chibnall's Torchwood scripting duties, Doctor Who and Torchwood executive producer Russell T Davies approached him with an offer to scribe an adventure for the parent series. Originally, this would have involved a generational space station, whose crew had grown up on board. However, Davies realised that although the new Doctor Who series had already enjoyed several episodes set on space stations, none had been set entirely on a spaceship, and so asked Chibnall to amend his storyline appropriately. It was suggested that Chibnall's script would be set in the same era as 2006's The Impossible Planet / The Satan Pit, and for a time, consideration was given to including the Ood from that story, although this did not survive to the drafting stage.

In developing the script, Chibnall and Davies hit on the notion of the episode proceeding in real time, much like the American adventure drama 24: each season consisted of twenty-four one-hour (with commercials) episodes, which collectively told the story of events occurring on a single day. The intensity of Chibnall's adventure would be accelerated by having the events actually occur over precisely its forty-two minute runtime, and Davies indicated that it should therefore simply be called 42.

The masks worn by the possessed crewmembers were inspired by the Marvel Comics superhero Cyclops

For the masks worn by the possessed crewmembers, Chibnall was inspired by the Marvel Comics superhero Cyclops, whose visor controlled the release of destructive energy beams from his eyes. Chibnall also drew on the 1992 film The Last Of The Mohicans for the scene in which the Doctor promises to rescue Martha from the escape pod. Riley's last name was Kincade at one point, but became Vashtee when Davies used Kincade as Brannigan's middle name in Gridlock; the name Riley itself was actually that of Chibnall's godson. Ashton was originally called Motta, but it was felt that this would sound too similar to “Martha”. Also amended was the name of the solar system in which 42 took place: Chibnall had written this as the Peony system, but it became the Toraji system following concerns that the original name would be misheard as “penis”.

Chibnall decided to call the spaceship at the centre of the story the SS Icarus, after the Greek myth about a boy who could fly using an invention of his father's, but who perished when he soared too close to the Sun. A major set piece involved the Doctor scaling the exterior of the Icarus, until fears were raised about the expense it would incur. Chibnall simplified this to the Doctor's efforts to reach the lever outside the airlock. In October, Davies asked Chibnall to add the material involving Francine Jones and the agent of Mr Saxon, who had been introduced in The Lazarus Experiment, the previous story to be broadcast.

42 was paired with Utopia to form the seventh production block of Doctor Who's 2007 season. The director was Graeme Harper, who had handled both Rise Of The Cybermen / The Age Of Steel and Army Of Ghosts / Doomsday the year before. For a time, it was thought that Utopia, despite being scheduled for transmission later in the season, would actually be filmed before 42, but this plan was scuppered when its principal guest star, Derek Jacobi, proved unavailable.

Instead, 42 began recording with three days at the Upper Boat Studios, spanning January 15th to 17th, 2007. This involved material in the TARDIS, the escape pod and the airlock, with the designers and set builders having worked feverishly to meet the new production dates. Greenscreen work of the McDonnells ejected from the airlock was also completed. By now, David Tennant had recovered from the voice issues which had plagued him during the making of Blink and Human Nature / The Family Of Blood during the past month. The only major location used for 42 was the St Regis Paper Company mill in Caldicot, Monmouthshire. A week was spent there from January 18th to 25th, pausing only on the 21st; scenes recorded included those in the locker room, the anteroom, the engineering area, and especially each section of the central corridor.

The SS Pentallian was named for a spaceship drive mentioned in 1975's Revenge Of The Cybermen

Cast and crew then returned to Upper Boat for the medcentre sequences, filmed on January 26th, 29th and 30th. On the latter day, the control panels for venting the engines were set up at the old NEG glass site at Trident Park in Cardiff Bay. Various inserts for 42 were taped at Upper Boat on February 8th and 9th. Around this time, it was discovered that Sunshine -- a science-fiction film with a solar theme due for release in the UK in April -- featured a spaceship called the Icarus II. It was agreed that the name of the craft in 42 could be changed through altered graphics, editing and dubbing without undue difficulty, and so Davies rechristened it the SS Pentallian, after the Pentalion drive mentioned in 1975's Revenge Of The Cybermen.

Material involving Francine Jones was recorded at a house in Penarth on February 20th. By this time, the agent of Mr Saxon had become a woman, as Bertie Carvel -- who had played the “Mysterious Man” in The Lazarus Experiment -- was committed elsewhere. Production concluded with additional pick-up shots, taped at Upper Boat on March 1st, 6th and 13th. Unfortunately, an error was made in compiling the credits for 42, which saw Matthew Chambers listed as playing “Hal Korwin” rather than Korwin McDonnell.

For the first time since Doctor Who had returned in 2005, a mid-season transmission break was scheduled between The Lazarus Experiment and 42 due to the broadcast of the Eurovision Song Contest on May 12th. Nonetheless, Doctor Who still made headlines on that day when Kylie Minogue confirmed that she would be appearing in the 2007 Doctor Who Christmas special. In fact, Minogue's character, Astrid Peth, would be filling the role of the companion in that story, as it was planned that Martha Jones would leave the Doctor at the season's end (although her character was to return in some 2008 adventures, as well as episodes of the second season of Torchwood). Originally an actress in her native Australia -- appearing in programmes such as Neighbours -- Minogue had gone on to greater international fame for her music career, which included hits like Can't Get You Out Of My Head and the 1988 remake of The Loco-Motion. Throughout, Minogue had continued to act in feature films such as Bio-Dome, Moulin Rouge! and the animated The Magic Roundabout (released as Doogal in North America). Most recently, Minogue had made headlines for her successful recovery from breast cancer.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #17, 22nd November 2007, “Episode 7: 42” by Andrew Pixley, Panini Publishing Ltd.