The TARDIS brings the Doctor, Sarah and Harry to the Nerva Beacon in the far future, where the remnants of humanity have been placed in suspended animation because of the risk of deadly solar flares on Earth. The humans have overslept by millennia, however, due to the incursion of the insect-like Wirrn. More Wirrn are gestating within Noah, the Beacon's leader, and as Noah starts to succumb to the alien influence, the human race faces imminent extinction..

The first storyline considered for the Fourth Doctor was “Space Station”, submitted by Christopher Langley on December 30th, 1973. New Doctor Who script editor Robert Holmes commissioned this four-part story on January 24th, 1974. As Langley worked on his scripts, Holmes and incoming producer Philip Hinchcliffe conceived the cost-saving notion of using the same sets for a second serial; this would become Revenge Of The Cybermen. Shortly thereafter, it was also decided that the studio-bound “Space Station” would be made by the same crew as a two-part location-only adventure, The Sontaran Experiment, which would act as something of a coda to Langley's story. “Space Station” was therefore rapidly becoming a lynchpin of Season Twelve.

Unfortunately, by the end of May, it was clear that Langley's scripts were unacceptable and a replacement would have to be found. With this story -- Serial 4C -- now burdened by several plot and structural requirements, Holmes turned to John Lucarotti on the recommendation of former script editor Terrance Dicks. Dicks had worked with Lucarotti on the short-lived science-fiction drama Moonbase 3, but Lucarotti's affiliation with Doctor Who went back more than a decade; he had last written The Massacre Of St Bartholomew's Eve in 1965. Lucarotti was commissioned to write episode one of The Ark In Space on June 5th, with the contract for the other three installments following the day after.

Lucarotti's adventure concerned a massive space ark carrying cryogenically-frozen humans, which has been invaded by the Delc. The Delc are fungi, grown from spores floating in space. The primary Delc take the form of floating heads, while their servitors appear as headless bodies. The Delc are impervious to most harm because any impact just causes the release of more spores. Fortunately, the Doctor eventually discovers that the Delc are susceptible to electrocution, and ultimately knocks the primary Delc out into space with a golf club. Having spent so long away from Doctor Who, Lucarotti hewed to the early-Sixties tradition of giving each episode its own title. Playfully, Lucarotti used only one-word names, each time ending in “ball” (such as “Golfball” for the final installment).

Unfortunately, contact between Lucarotti -- who wrote from his houseboat moored at the French island of Corsica in the Mediterranean -- and Holmes was sporadic, a situation further hampered by a postal dispute. When Lucarotti's scripts arrived in July, they were once again met with disapproval. With time growing short, it was decided that Holmes would rewrite The Ark In Space himself, although Lucarotti would be paid in full for his work; Hinchcliffe secured retrospective approval for this on October 8th. Completing the new scripts in just eighteen days, Holmes discarded many of Lucarotti's ideas, although he preserved the basic premise of a space ark invaded by an alien intelligence (now the insectoid Wirrn rather than the fungal Delc).

Since both The Ark In Space and The Sontaran Experiment were effectively being made as one large production at the start of Doctor Who's twelfth recording block, they shared the same director: Rodney Bennett. Bennett disliked the end of Holmes' story, which saw Noah draw the Wirrn out into the depths of space, away from the Ark. Bennett wanted the Wirrn to meet a more decisive end, and suggested they could be dissolved by water. Holmes rejected this notion, and instead amended episode four to have Noah blow up the shuttle with the Wirrn aboard. Bennett also ignored Holmes' original idea that Vira should be black (and possibly Haitian), instead casting the caucasian Wendy Williams in the role.

Work on The Ark In Space began with a day's model filming at the Television Centre Puppet Theatre on October 16th. This was followed by a two-day studio block in BBC Television Centre Studio 3, on Monday the 28th of October and Tuesday the 29th. The first day saw the recording of part one, as well as scenes in the control rooms and the cryogenic chamber for part three. Episode two was taped on the second day, alongside material from the beginning of the third installment.

Cast and crew relocated to TC1 on Monday, November 11th for the start of the second studio block. The majority of the outstanding episode three scenes were completed on this day, together with part four sequences set in the second control room and the transom. Tuesday the 12th was concerned with the rest of the fourth installment, in addition to part three material in the second control room, the cryogenic chamber, and the access chamber.

Hinchcliffe intended to take Doctor Who in a more mature direction, but the producer was aware that this new approach had to be instituted carefully, given the programme's chiefly juvenile audience. As a result, Hinchcliffe decided to cut one significant sequence from The Ark In Space following consultations with Head of Serials Bill Slater. This was a longer version of the encounter between the Doctor, Vira and the half-Wirrn Noah. In the excised material, Noah was to describe the dual ecstasy and torment of becoming a Wirrn, culminating with a plea to Vira to kill him, which she is unable to bring herself to do. Both Hinchcliffe and Slater feared that this scene would prove too disturbing to children.

Although The Ark In Space was made after The Sontaran Experiment, it was planned all along to exchange the two stories in the transmission order. The Ark In Space therefore came after Robot, which had been held over from the end of the previous recording block. Any fears that the public might react badly to Tom Baker's new Doctor following Jon Pertwee's five-year tenure were quickly allayed, as episode two (broadcast on February 1st, 1975) drew 13.6 million viewers to finish fifth amongst UK programmes for the week. This was the highest chart position ever achieved in the history of Doctor Who, and was just the first sign of the incredible success Baker would bring to the show.

• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• Doctor Who Magazine #218, 26th October 1994, “Archive: The Ark In Space” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “You Ain't Seen Nothing Yet” by Andrew Pixley, Panini Publishing Ltd.
• In-Vision #2, February 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.