The TARDIS materialises aboard the Nerva Beacon, a space station carrying the remnants of humanity in suspended animation. The Earth was abandoned before it was scoured by deadly solar flares, but mechanical failure has caused the survivors to oversleep by millennia. The Doctor, Sarah and Harry begin reviving the humans, including their leader, Noah. Soon, however, one of the crew is found to be missing, and the Doctor realises that Nerva has suffered an incursion by the insect-like Wirrn. When Noah is infected by the Wirrn and begins to succumb to their hostile influence, mankind faces imminent extinction.

The first storyline considered for the Fourth Doctor was “Space Station”, submitted by Christopher Langley on December 30th, 1973. As Langley worked on his scripts, they became a lynchpin of the plans being formulated by producer Philip Hinchcliffe and script editor Robert Holmes for Doctor Who's twelfth season. The pair had decided to make more effective use of the programme's budget by setting two stories in the same location at different times. As such, the sets required for “Space Station” would also be utilised in a second serial which, by May 1974, was intended to be Gerry Davis' Revenge Of The Cybermen.

Unfortunately, by the end of May, it was clear that Langley's scripts were unacceptable and a replacement would have to be found. Since the new storyline would now burdened by several plot and structural requirements, Holmes turned to John Lucarotti, who was recommended to him by 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 next day.

John Lucarotti's version concerned a space ark invaded by the Delc, fungi grown from spores floating in space

Lucarotti's adventure concerned a massive space ark carrying cryogenically-frozen humans, which had been invaded by the Delc. The Delc were fungi, grown from spores floating in space. The primary Delc took the form of floating heads, while their servitors appeared as headless bodies. The Delc were impervious to most forms of harm because any impact just caused the release of more spores. The Doctor eventually discovered that the Delc were susceptible to electrocution, and ultimately knocked the primary Delc out into space with a golf club. Lucarotti gave each episode its own title, unaware that Doctor Who had abandoned this practice in 1966.

Contact between Holmes and Lucarotti -- who wrote from a houseboat moored at the French island of Corsica in the Mediterranean -- was sporadic, a situation further hampered by a postal dispute. When Lucarotti's scripts arrived in July, they too were met with disapproval, having evolved into a sophisticated narrative unsuitable for Doctor Who. With time growing short, it was decided that Holmes would have to perform a wholesale rewrite of The Ark In Space; Lucarotti would not be credited on the broadcast episodes, but he would nonetheless be paid in full for his work. Hinchcliffe secured retrospective approval for this arrangement 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).

As a further cost-saving measure, Hinchcliffe and Holmes had decided to pair The Ark In Space with The Sontaran Experiment, in which the Doctor and his companions investigated whether the abandoned Earth was safe for the sleepers aboard the Nerva Beacon. The two serials would effectively be made as a single production, with The Ark In Space taped solely in the studio and The Sontaran Experiment made entirely on location. As such, they would share the same director: Rodney Bennett. Bennett disliked the end of Holmes' story, in which Noah drew the Wirrn out into the depths of space, away from the ark. Bennett wanted the Wirrn to meet a more decisive end, and suggested the revelation that they could be dissolved in water. Holmes rejected this notion, and instead amended Episode Four to add Noah blowing up the shuttle with the Wirrn aboard. Bennett and Holmes also clashed in their conception of Vira, the chief medtech aboard the Nerva Beacon. Holmes' scripts indicated that she should be black -- and possibly Haitian -- but Bennett instead cast Wendy Williams, a white actress, in the role.

Work on The Ark In Space began on October 16th with a day's model filming at the BBC Television Centre Puppet Theatre in White City, London. As usual, the serial's studio schedule was divided into fortnightly two-day blocks, on Mondays and Tuesdays. The first took place on October 28th and 29th, in BBC Television Centre Studio 3. The first day saw the recording of Episode One, as well as the opening moments of Episode Two and shots from the perspective of the Wirrn Queen for Episode Three. The rest of Episode Two was taped the next day.

Plans to complete the early scenes of Episode Three on October 29th were scuppered by the slow pace of production

Bennett had also intended to complete the early scenes of Episode Three on October 29th, since these were the last sequences in which the Wirrn mutation was confined to Noah's hand. However, these plans were scuppered by the slow pace of production, and the material was shifted to the second recording block, which took place in TC1 on November 11th and 12th. Because Kenton Moore had to be made up as a more fully-mutated Noah for the rest of Episode Three, the delayed material could not be completed on the Monday. Instead, this day also took in sequences from Episode Four set on the transom and in the second control room. The rest of Episode Four was taped on the Tuesday, at which time the opening scenes of Episode Three were finally recorded.

One of Hinchcliffe's plans for Doctor Who was to take it in a more mature direction, but the producer was aware that he had to do so cautiously, given the programme's chiefly juvenile audience. As a result, Hinchcliffe decided to cut one significant sequence from The Ark In Space Episode Three after consulting with Bill Slater, the BBC's Head of Serials. This was a longer version of the encounter between the Doctor, Vira and the half-Wirrn Noah. In the excised material, Noah described the dual ecstasy and torment of becoming a Wirrn, and pleaded with Vira to kill him, although she was unable to do so. Both Hinchcliffe and Slater feared that this scene would prove too disturbing for children.

Although The Ark In Space was made after The Sontaran Experiment, it was always planned to be broadcast first. The Ark In Space therefore came after Robot, the debut story for Tom Baker's Fourth Doctor, which had been held over from the previous recording block. It was the first serial to which the new Doctor Who title sequence was added, since the editing of Robot was only subsequently completed. The titles appended to The Ark In Space featured a unique colour scheme, with a predominantly green tint instead of blue.

The BBC Saturday night schedule underwent some changes during the transmission of The Ark In Space. Up to the broadcast of Episode One on January 25th, 1975, Doctor Who had been preceded by the children's show Shari Lewis and a news update, but the former was replaced by the animated Tom And Jerry the following week. The same evening marked the season finale for Bruce Forsyth And The Generation Game, into which Doctor Who led. As of Episode Three on February 8th, Doctor Who was instead followed by The Wonderful World Of Disney.

Meanwhile, any fears that the public might react badly to Baker's new interpretation of the Doctor were quickly allayed. On February 1st, The Ark In Space Episode Two drew 13.6 million viewers to finish fifth amongst all UK programmes for the week. This was the highest chart position yet achieved in the history of Doctor Who, and was just the first sign of the incredible success Baker would help inspire.

• 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.
• Doctor Who: The Ark In Space Special Edition DVD Production Subtitles by Martin Wiggins (2013), 2|entertain.
• Doctor Who: The Complete History #22, 2015, “Story 76: The Ark In Space”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• In·Vision #2, February 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.