The TARDIS lands on a planet where, every year, the two brightest Gond youths disappear into the bowels of a machine to join their people's gods. But the Doctor, Jamie and Zoe discover that the “gods” are really crystalline aliens called the Krotons, who are feeding on the mental energies of the children. And the Doctor and Zoe are next on the menu.

Robert Holmes parlayed an early career as a policeman into a job as a court reporter and, from there, into wider writing circles. He started scripting for television in the late Fifties, contributing to shows like Ghost Squad and Doctor Finlay's Casebook. In early 1965, Holmes submitted an idea for a science-fiction serial to BBC Head of Drama Serials Shaun Sutton. Sutton indicated to Holmes that his idea wasn't really the sort of material the Corporation was looking to broadcast at the time, but suggested Holmes might reconceive his submission as a Doctor Who adventure.

Holmes met with then-story editor Donald Tosh on April 23rd, 1965. Subsequently, the writer quickly reworked his idea into a storyline entitled “The Trap” in which the Doctor and his companions are unknowingly recruited by the dormant crew of a spaceship who need some of them to help pilot their vessel. Tosh responded in May with concerns that the sleepers' robotic servants were too similar to the Mechonoids, which would shortly feature in The Chase. Shortly thereafter, Holmes began working on the drama series Private Eye, and “The Trap” fell by the wayside.

Three years later, Holmes was cleaning out some old files when he came across the unused storyline. Feeling it still had value, Holmes resubmitted “The Trap” to Doctor Who producer Peter Bryant. The pair met to discuss the idea shortly thereafter, joined by current story editor Derrick Sherwin and his assistant, Terrance Dicks. Holmes was commissioned to write a new story breakdown on May 31st, 1968 under the title “The Space-Trap”. On June 25th, scripts for the serial -- with the title amended slightly to drop the hyphen -- were requested. It was thought that “The Space Trap” might be scheduled for late in Season Six (possibly as Serial YY), and so the story was given to Dicks to edit on his own, as part of his training.

Meanwhile, problems were arising on an earlier adventure for the season. Serial WW was originally intended to be “The Dreamspinner” by Paul Wheeler; this was abandoned on April 9th, apparently after the script for episode one proved unsatisfactory. An unknown story was then assigned the slot before being replaced by Dick Sharples' comedic “The Amazons” at the end of April. “The Amazons” was renamed “The Prison In Space” in June; at the same time, Sharples was told that Serial WW would see the departure of Frazer Hines' Jamie, as Hines now felt it was time to move on from Doctor Who. Sharples was asked to amend his plot suitably, and also to introduce a new companion named Nik, whom Bryant and Sherwin had conceived.

Despite the lack of pressure to deliver his scripts, Holmes wasted little time completing “The Space Trap”. Only minor changes occurred: Eelek was originally called Avrik, while the Dynotron became the Dynotrope. “The Space Trap” was basically complete by the middle of August, despite the fact that it was not planned to enter production until the new year.

In September, however, Hines changed his mind about leaving Doctor Who after Patrick Troughton asked the actor to remain on the programme until his own departure in the spring. Plans to introduce Nik were therefore dropped, and Sharples was asked to redraft “The Prison In Space”. Unfortunately, these rewritten scripts met with disapproval from the production team and David Maloney, the director assigned to Serial WW (Maloney had recently worked on The Mind Robber at the end of the last recording block). Sharples balked at the idea of performing even more work on a story which he felt he had completed to its original specifications.

Discussions ensued between Sharples and the BBC with a view to settling the disagreement, but it quickly became apparent that “The Prison In Space” would not be ready for the start of production on Serial WW in early November. Maloney was given a copy of the completed scripts for “The Space Trap” and although he did not feel Holmes' story was particularly good either, he indicated that he would be willing and able to direct it in place of “The Prison In Space”. Bryant concurred, and on October 7th “The Space Trap” became the new Serial WW; it was retitled The Krotons on October 30th. “The Prison In Space”, meanwhile, was finally abandoned on the 15th.

A few weeks earlier, on September 26th, Patrick Troughton had received a new contract for twenty-two episodes spanning Serial WW to the six-part Serial ZZ; it was planned that the actor would leave Doctor Who upon its expiry. On October 17th, Wendy Padbury's services were secured for Serials WW, XX and YY.

A suggestion made by The Enemy Of The World director Barry Letts the previous year resulted in a major alteration to Doctor Who's production philosophy for its sixth recording block. The number of episodes which would be made during the block was significantly reduced. This, in turn, meant that filming for a serial no longer overlapped with the studio weeks of the preceding story, which had always necessitated taking the regular cast out of rehearsals on certain days. An extra week would now pass between consecutive serials during which such filming could be completed. This change was particularly embraced by Troughton, who had found the pace of making Doctor Who to be very demanding.

Consequently, location work on The Krotons took place on November 10th and 11th, with the West of England Quarry and the Tank Quarry in Malvern, Worcestershire standing in for the Waste Land. Three days at the Ealing Television Film Studios then followed from the 12th to the 14th; the last of these was used for model shots.

Studio work got under way on November 22nd. As usual, all four episodes of The Krotons were recorded on successive Fridays in Lime Grove Studio D. Unusually, however, episode one was captured directly on 35mm film rather than videotape. For the taping of part three on December 6th, Beta was assigned a line of dialogue originally intended for another Gond character, probably to eliminate the expense of an extra speaking part. Unfortunately, this meant that the finished installment made it appear as if Beta was simultaneously in the Underhall and his laboratory. The Krotons was completed on December 13th.

• Doctor Who: The Handbook: The Second Doctor by David J Howe, Mark Stammers and Stephen James Walker (1997), Virgin Publishing, ISBN 0 426 20516 2.
• 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 #318, 26th June 2002, “Archive: The Krotons” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #4, 4th June 2003, “Paradise Lost” by Andrew Pixley, Panini Publishing Ltd.