The TARDIS takes the Doctor, Steven and Dodo to a space ark in the far future, which is carrying humanity from the doomed Earth to their new home, the planet Refusis. Dodo has a cold, however, for which the humans and the subservient alien Monoids have no immunity. The Doctor must find a cure for the common cold, and, centuries later, stop a revolution by the Monoids brought about by the ramifications of the plague.

Although it was writer Paul Erickson who formally received the commission for The Ark on May 27th, 1965, the ideas for the story actually originated with incoming producer John Wiles, who envisaged a Doctor Who serial set on an enormous spaceship. Wiles discussed this image with story editor Donald Tosh and Tosh involved Erickson, with whom he had worked on Compact; Erickson's credits also included series such as The Saint and several feature films. Tosh and Erickson developed the storyline between them, from which Erickson then composed the scripts.

The Ark was originally designated Serial Y. By early June, however, it was known that the six-part Serial V would be expanded into the mammoth twelve-episode The Daleks' Master Plan. To account for the resulting rearrangement of the schedule for Doctor Who's third season, The Ark would now be Serial X. Shortly before the story entered production in January 1966, Erickson's agent approached the BBC about amending the writing credit for The Ark to list both Erickson and his wife, Lesley Scott. Copyright on the four scripts, however, would remain solely with Erickson. This would be the lone contribution to Doctor Who for both Erickson and Scott, although Erickson did novelise the story for WH Allen prior to his death, following a stroke, on October 27th, 1991.

Meanwhile, there had been considerable turmoil in the Doctor Who production office during the months Erickson had been writing The Ark. This had culminated in December with the resignations of both Wiles and Tosh. Tosh was replaced by Gerry Davis beginning with the final episode of The Massacre Of St Bartholomew's Eve, the serial which preceded The Ark into production. Some stability was introduced on February 15th, when Hartnell received a contract extension to keep him on the show at least through Serial CC -- the last of the current production block. On the 24th, Purves' contract was also renewed, but only with the understanding that his tenure on Doctor Who would end after another twelve episodes, at the conclusion of The Savages.

The director assigned to The Ark was Michael Imison, also working on his only Doctor Who story. Imison had started at the BBC as a story editor before becoming a director, with credits including Compact. Production began on January 24th at the Ealing Television Film Studios with the miniaturisation of the Guardian. A further four days were spent at Ealing between January 31st and February 3rd. The first three days concentrated on the Monoid battle on Refusis, while the final day was used for the material in the Ark jungle. The latter involved the use of a baby Indian elephant, named Monica, in addition to a variety of other exotic animals. Bizarrely, Monica was forced to spent the previous night in her van outside Imison's home after the van's driver had been refused permission to park at Ealing.

It was around this time that Wiles' successor, Innes Lloyd, began trailing the producer. Although Wiles would still be credited on The Ark, Lloyd would have effectively replaced him by the end of recording. Wiles, who had never been comfortable in the producer's chair, returned to writing and directing. Most of his efforts were directed toward the theatre, although he also earned credits on programmes like Out Of The Unknown and Paul Temple (for which Erickson would later write as well). Wiles had been ill for some time when he passed away on April 5th, 1999.

The Steel Sky was recorded on February 18th in Riverside 1, the studio home for the entire serial. Unfortunately, Imison vastly overspent on this episode, meaning that savings would have to be made on the following serial, The Celestial Toymaker. This would be Jackie Lane's first full Doctor Who serial as Dodo after the character debuted in the final episode of The Massacre Of St Bartholomew's Eve. In the intervening week, Lane had been asked to tone down the pronounced Cockney accent she was employing for Dodo due to concern about a character not speaking proper “BBC English”. This was challenging for Lane because Erickson's scripts were written with Dodo's original speech pattern in mind; nonetheless, her accent would all but disappear over the next few recording sessions.

The Ark followed the usual Doctor Who production pattern of taping each episode on successive Fridays, concluding with The Bomb on March 11th. For this installment, Imison broke with the typical Doctor Who recording scheme of taping scenes in story order, the first time this had been attempted on such a large scale on the programme. Nonetheless, it would prove to be a sign of the coming changes to the way Doctor Who episodes were made.

• Doctor Who: The Handbook: The First Doctor by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 0 426 20430 1.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 1 85227 420 4.
• Doctor Who Magazine #228, 2nd August 1995, “Archive: The Ark” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #7, 12th May 2004, “I'm Into Something Good” by Andrew Pixley, Panini Publishing Ltd.