On the planet Marinus, the scientist Arbitan locks the Doctor, Susan, Ian and Barbara out of the TARDIS in a desperate bid to convince them to embark on a quest to find a lost set of keys. These keys power the Conscience of Marinus, a mighty computer which is Marinus' last hope against the onslaught of Yartek and his evil Voord. But many dangers lie between the companions and each key: hypnotic monsters, killer plants, ice zombies, and finally a charge of murder.

On September 2nd, 1963, writer Malcolm Hulke responded to a July invitation from Doctor Who story editor David Whitaker to contribute to the fledgling series. Hulke had co-authored the Pathfinders science-fiction serials for ABC in 1960 and 1961; these had been produced by Doctor Who creator, and current BBC Head of Drama, Sydney Newman. Hulke sent Whitaker outlines for two six-part serials. The first was an historical adventure set around the time of the departure of the Romans from Britain, in the early fifth century AD. It was originally this submission which was pursued by Whitaker and producer Verity Lambert, having been allocated the sixth spot in the running order (after 100,000 BC, The Daleks, Marco Polo, Robert Gould's untitled “miniscules” story, and Anthony Coburn's “The Masters Of Luxor”).

However, by mid-October, plans had changed and Hulke's other offering, entitled “The Hidden Planet”, had replaced the historical on the schedule. This was a “sideways” adventure set on a planet which orbited the Sun diametrically opposite the Earth, whose ruler was a double of Barbara. “The Hidden Planet” was commissioned on December 2nd, by which time the insertion of Inside The Spaceship into the running order between The Daleks and Marco Polo had pushed Hulke's serial back to seventh.

By the dawn of 1964, however, unresolved problems with both Gould's and Coburn's serials meant that “The Hidden Planet” would now be transmitted fifth, immediately after Marco Polo. Unfortunately, as Hulke began delivering his scripts, it became clear to Lambert and Whitaker that they were not acceptable in their present forms. With less than two months left before the fifth serial was due into production, this meant that a replacement adventure would have to be commissioned very quickly. By mid-April, “The Hidden Planet” was being considered for the second spot of Doctor Who's second recording block; around the end of July, a rewritten five-part version was a contender to be made as the first serial of that block. Finally, on September 24th, Whitaker asked Hulke to cease working on “The Hidden Planet” altogether. He believed that too much work would be needed to account for the departure of Susan, and also cited the adventure's lack of monsters -- now thought to be a popular component of Doctor Who's non-historical stories.

Meanwhile, on September 24th, 1963, Terry Nation -- who had just completed work on The Daleks -- was commissioned to write a seven-part historical entitled “The Red Fort”, set during the Indian Mutiny of 1857. This was planned to be the seventh story of the season, immediately following “The Hidden Planet” at that point. Nation, however, had not particularly enjoyed writing The Daleks and it appears he did little work on “The Red Fort”. On January 21st, Whitaker met with Nation and it was agreed that “The Red Fort” should be abandoned. In its place, Nation would hastily write a six-part story to replace “The Hidden Planet”; Whitaker was aware of Nation's ability to write quickly following his prompt delivery of scripts for The Daleks. Because the serial would have to be completed in just four weeks, Whitaker offered to help Nation with ideas, and the pair agreed that the story should be made up of several small adventures to aid the process.

Nation's serial soon became known as The Keys Of Marinus. He developed more background than was made explicit onscreen in the finished production. The Voord were alien invaders who took advantage of the people of Marinus, rendered vulnerable by the pacifying effects of the Conscience. The Conscience was then deactivated to allow the Marinians to fight the Voord, and over the centuries, the time they spent on Marinus meant that the Voord, too, could now be affected by the machine. Therefore, Arbitan despatched agents to recover the keys which would reactivate the Conscience and allow him to finally defeat the invaders.

The opening and closing episodes were entirely Nation's creations. The second and fifth installments arose from discussions between Nation and Whitaker. Whitaker came up with the idea of a house full of traps for part three; Nation largely transferred this to an exterior setting because the first two episodes had already mostly taken place indoors. It was Whitaker who suggested a nordic environment for part four, to contrast with the jungle setting of the previous installment. Amongst later changes made to Nation's scripts was the removal of a TARDIS sequence from episode one, The Sea Of Death. Here it was revealed that the reason the Doctor and Susan had been on Earth in 1963 at the start of 100,000 BC was because the Doctor had visited the British Broadcasting Corporation to get help repairing the colour scanner in the TARDIS, which was showing only monochrome images. He had been in such a bad mood upon his return to the TARDIS because the BBC had been “infernally secretive”!

The director assigned to The Keys Of Marinus, designated Serial E, was John Gorrie. Gorrie had begun his career as an actor before becoming an assistant floor manager at the BBC. He took the BBC's director's course in early 1963 and had recently completed work on the soap opera Compact and the anthology series Suspense when Lambert requested him for Doctor Who. Gorrie was very reluctant to work on the programme, having aspired to direct classics.

Model filming for The Keys Of Marinus took place in early March at the Ealing Television Film Studios. This included the first-ever shot of the TARDIS materialising, a model effect of the Ship landing on the beach. Meanwhile, it had been decided that the four series regulars should be given two-week breaks throughout the remainder of the season, now that their characters had been well-established in the initial four adventures. First to go on holiday was William Hartnell, so in mid-March Whitaker restructured the third and fourth episodes to eliminate the Doctor. Originally, he accompanied Susan and Sabetha in The Snows Of Terror (and his ring would have been found in Vasor's hut along with the travel dials and Conscience keys). Instead, he would now leap ahead to Millenius, the location of the final key.

Recording for The Sea Of Death took place on Friday, March 20th, at Lime Grove Studio D; as usual, the remaining five episodes were taped on consecutive Fridays. Hartnell was absent on both April 3rd and 10th while he enjoyed his vacation.

On March 26th, Whitaker was forced to defend the third installment, The Screaming Jungle, from a charge of plagiarism levied by Robert Gould. On February 4th, Whitaker and Gould had agreed to abandon his “miniscules” adventure due to insurmountable difficulties with the scripts. Gould suggested that instead he might write a story about a planet where plants treat people the way that the people on Earth treat plants. Lambert was concerned that this might appear too similar to the John Wyndham novel The Day Of The Triffids, which had been made into a film in 1962. On February 9th, Gould informed Whitaker that he had decided against developing the concept. Gould was now concerned that The Screaming Jungle -- with its hostile plant life -- had been inspired by his idea, but Whitaker was able to satisfy Head of Serials Donald Wilson that this was not the case.

The movie rights to Nation's first three serials -- The Daleks, The Keys Of Marinus and The Dalek Invasion Of Earth -- were all optioned by Amicus. However, it seems that the rights to The Keys Of Marinus were eventually dropped, most likely due to the absence of the Daleks from the story.

• 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 #310, 14th November 2001, “Archive: The Keys Of Marinus” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #7, 12th May 2004, “Do You Want To Know A Secret?” by Andrew Pixley, Panini Publishing Ltd.