The Doctor and Martha find themselves trapped aboard a spaceship which is spiralling into a sun. Its main engines have been sabotaged by Korwin, the husband of Captain McDonnell, who has suddenly gone berserk. The Doctor soon discovers that Korwin's entire biological make-up is changing. His eyes start to burn with a terrible light, incinerating at a glance, and he begins to prowl the ship -- killing some crewmembers and infecting others. The time travellers have just forty-two minutes to avert disaster... but the Doctor is succumbing to the same horrible transformation.

With Doctor Who having enjoyed enormous success following its return to television in 2005, it wasn't long before the notion of a spin-off series was being discussed. This became Torchwood, an older-skewing drama set in Cardiff which utilised various elements established during Doctor Who's twenty-seventh and twenty-eighth seasons. The head writer for the first season of Torchwood was Chris Chibnall, whom executive producer Russell T Davies knew to be a lifelong fan of Doctor Who. In July 2006, midway through his work on Torchwood, Chibnall was offered the opportunity to write a script for the parent show's twenty-ninth season. It was suggested that this might be a sequel to 2006's The Impossible Planet / The Satan Pit, revisiting the characters of Zachary Cross Flane and Ida Scott, as well as the Ood servitor race.

Because of his responsibilities to Torchwood, Chibnall's adventure would be amongst the last scripts to be developed for 2007. At any early stage, it was intended to be set on a research space station which had been studying the same sun for generations; the crew had grown up on board. A major plot twist would be the revelation that the sun harboured sentient lifeforms, although Davies was concerned that the crew's ignorance of the presence of these entities might seem ridiculous. Furthermore, budgetary considerations ultimately required a smaller-scale narrative, without the connections to The Impossible Planet / The Satan Pit. Davies noted that, although there had been several episodes of twenty-first-century Doctor Who set on space stations, none had taken place entirely on a spaceship. This environment would be cheaper to realise, and so Chibnall was asked to amend his storyline accordingly, with the blue-collar crew now having only just happened upon the sun.

The title 42 was an allusion to 24, and also nodded to The Hitchhiker's Guide To The Galaxy

A key hook for Chibnall's script came from Davies, who proposed that it should proceed in real time. Amenable to the idea, Chibnall thought in terms of the American adventure drama 24, starring Kiefer Sutherland. Since its debut in 2001, the twenty-four episodes of each 24 season had chronicled the events of a single day, at one hour per installment. The action of Chibnall's narrative would likewise be intensified by having it take place precisely within the programme's forty-two-minute runtime. As such, Davies felt that the adventure should simply be called 42. Chibnall liked this, both because of its obvious allusion to 24, and also because it nodded to The Hitchhiker's Guide To The Galaxy by Douglas Adams, who had contributed to Doctor Who in the late Seventies. Originally a radio play before being adapted into various other media, the science-fiction classic had revealed the number 42 to be the Answer to the Ultimate Question of Life, the Universe, and Everything.

Positioned as the seventh episode of Season Twenty-Nine -- precisely at the midway point of the schedule -- Davies wanted 42 to mark the stage at which Martha Jones stopped being a guest aboard the TARDIS and became an established companion. This would be particularly important because the next story, Paul Cornell's Human Nature / The Family Of Blood, would literally see the Doctor entrust Martha with his very identity. As such, Chibnall was asked to give her a pivotal role in his adventure. He originally proposed that she should become infected by the solar entity, but Davies felt that it would be more effective if the Doctor suffered this fate, with Martha playing a crucial part in resolving the situation.

For the masks worn by the infected 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 upon the 1992 film The Last Of The Mohicans for the scene in which the Doctor promised to rescue Martha from the escape pod. Korwin's death was first portrayed as the result of a calculated decision by Captain McDonnell, but this was deemed too ruthless. Chibnall duly revised the events so that McDonnell was left with no other choice. Vashtee was originally called Kincade, until Davies decided to use it as Brannigan's middle name in the same season's Gridlock. His first name, Riley, was shared with Chibnall's godson. Motta, the second crewmember to be infected, became Ashton due to concerns that the original name would sound too much like “Martha”. Also amended was the nomenclature for the solar system in which 42 took place: originally the Peony system, it became the Toraji system following concerns that it might be misheard as “penis”.

A major set piece would have involved the Doctor scaling the exterior of the spaceship

Chibnall decided to call the spaceship at the centre of 42 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. As a result, Chibnall replaced the sequence with the simpler action of the Doctor trying 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 preceding story, The Lazarus Experiment.

42 was paired with the season's eleventh episode, Utopia, to form Block Seven of the production calendar. 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. It was originally intended that 42 would be largely completed before Harper turned his attention to Utopia, but the order was reversed in mid-December. On January 5th, 2007, however, it was discovered that Derek Jacobi, the principal guest star in Utopia, would be unavailable for the revised filming dates. 42 was duly brought forward, forcing the design teams to work feverishly to get sets, props and costumes ready in time.

Under the hastily-revised schedule, recording for 42 began with three days at the usual Doctor Who studio space in Upper Boat, spanning January 15th to 17th. The first day saw cameras rolling on the TARDIS set, before Harper started on the sequences in the escape pod and the airlock. This work continued to the middle day. The last day at Upper Boat was devoted to the time-consuming effects sequences in which the Doctor emerged onto the hull of the spaceship, and Captain McDonnell and her husband were sucked out into the void. The only major location used for 42 was the St Regis Paper Company mill in Caldicot, with the actors employing various techniques to keep their breath from steaming in the chilly environment. Filming there began on January 18th, the first of two consecutive days devoted to scenes in the engineering area. This material was completed on the 20th, after which Harper taped action in the locker room, and then started on the various sections of the central corridor. It was back to the paper mill after a break on the Sunday, with sequences in the central corridor continuing to dominate the agenda from January 22nd to 25th. Part of the latter day also saw the completion of material in the anteroom where the TARDIS materialised.

Cast and crew then returned to Upper Boat for scenes in the med-centre, most of which were filmed on January 26th and 29th, with the weekend falling in between. Some additional med-centre footage was recorded on the 30th, after which Harper's team travelled to the old Nippon Electric Glass site in Cardiff, where the control panels for venting the spaceship's engines had been set up. Various inserts for 42 were then taped at Upper Boat on February 8th and 9th. Around this time, it was discovered that Sunshine -- a solar-themed science-fiction film directed by Danny Boyle, due for British release 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. As such, the vessel was rechristened the SS Pentallian, after the Pentalion drive mentioned in 1975's Revenge Of The Cybermen.

Material for 42 involving Francine Jones was recorded on February 20th at a house on Cwrt-y-Vil Road in Penarth. By this time, it was known that Bertie Carvel, who had played the “mysterious man” in The Lazarus Experiment, was committed elsewhere. Mr Saxon's agent in 42 therefore became a “sinister woman”, portrayed by Elize du Toit. This left only some additional pick-up shots, which were taped at Upper Boat on March 1st, 6th and 13th. In post-production, an unfortunate error was made in compiling the credits for 42. As a result, Matthew Chambers was listed as playing “Hal Korwin” rather than “Korwin McDonnell”.

Following a one-week hiatus due to the preceding Saturday's coverage of the Eurovision Song Contest, 42 aired on May 19th. Doctor Who was pushed to the later time of 7.15pm to accommodate the FA Cup final between Chelsea and Manchester United, which aired as part of Match of The Day Live that afternoon. There was also a change to the BBC One schedule immediately after Doctor Who, with The National Lottery Saturday Night Draws now inserted prior to Any Dream Will Do.

• Doctor Who Magazine #577, June 2022, “Setting The Story Straight” by Marcus Hearn and Emily Cook, Panini UK Ltd.
• Doctor Who Magazine Special Edition #17, 22nd November 2007, “Episode 7: 42” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #55, 2015, “Story 184: 42”, edited by John Ainsworth, Hachette Partworks Ltd.