Every year in the city of the Gonds, the two brightest youths are taken into the bowels of a strange machine to join their people's gods, known as the Krotons. When the TARDIS lands nearby, however, the time travellers discover that the young people are somehow drained of their mental energies, then secretly ejected from the machine and killed. As this discovery sparks revolution and counter-revolution, Zoe succumbs to her inquisitiveness and activates a Gond learning machine. When she, too, is summoned to serve the Krotons, the Doctor has no choice but to confront the mysterious gods himself.

In early 1965, Robert Holmes developed a Doctor Who storyline entitled “The Trap”, in which robot servitors tested the TARDIS crew to determine which of them should be selected to help pilot a crashed spaceship. When Holmes began writing for the drama series Private Eye, he ceased working on “The Trap”, and it was forgotten for three years. Holmes rediscovered it while cleaning out some old files and felt that it still had value, so he resubmitted it to Doctor Who producer Peter Bryant on May 20th, 1968. The pair met shortly thereafter, joined by current story editor Derrick Sherwin and his assistant, Terrance Dicks. Although Sherwin was unimpressed with Holmes' ideas, Dicks was more keen. It was therefore agreed that the story would be given to Dicks to edit on his own, as part of his training.

Holmes was commissioned to write a new story breakdown on May 31st, 1968 under the title “The Space-Trap” (although the hyphen would be included inconsistently during its development). By the time the scripts were requested on June 25th, it was thought that “The Space-Trap” might be scheduled for late in Season Six. Despite the lack of pressure to deliver his scripts, Holmes wasted little time in 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 expected to enter production until the new year.

The slot following The Invasion was originally to be filled by The Dreamspinner

Meanwhile, problems were arising with other stories. The slot following The Invasion was originally to be filled by “The Dreamspinner”. However, this was abandoned on April 9th, apparently after the script for its first episode proved unsatisfactory. The slot was then assigned to an unknown story, before it was replaced by Dick Sharples' comedic “The Prison In Space” at the end of April. In June, Sharples was told that his story would see the departure of Frazer Hines' Jamie, as Hines felt that two years on Doctor Who was enough. Sharples was asked to amend his plot to write out Jamie and introduce a new companion named Nik, whom Bryant and Sherwin had conceived.

In September, however, Hines changed his mind about quitting Doctor Who and decided that he would now leave alongside Patrick Troughton in the spring. Plans to introduce Nik were therefore dropped, and Sharples was asked to redraft “The Prison In Space” accordingly. Unfortunately, these rewritten scripts met with disapproval from the production team. Also dissatisfied was David Maloney, who had been assigned as director; 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 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 it was in competent shape to go before the cameras. Bryant concurred and, on October 7th, “The Space-Trap” took the place of “The Prison In Space” in the Doctor Who schedule. Holmes' serial was retitled The Krotons on October 30th; “The Prison In Space” was finally abandoned on October 15th.

Meanwhile, on September 26th, Patrick Troughton received a new contract which would take effect with the slot ultimately filled by The Krotons. The extension ran for a total of twenty-two episodes, and it was anticipated that the actor would then leave Doctor Who. On October 17th, Wendy Padbury's services were secured for The Krotons and the two subsequent serials.

The costume designer assigned to The Krotons was Bobi Bartlett. She decided to deviate from Holmes' vision of the title monsters as crystalline men, opting instead for a more robotic approach. Prop-builders Jack Lovell Ltd were tasked with constructing the Krotons but, in their haste to build the monsters, they did so to the wrong scale. As a result, the Krotons were larger and more unwieldy than Bartlett had intended, and a rubber skirt had to be added to hide the operator's legs, which would otherwise now be exposed.

The Krotons was the first serial to benefit from the new production pattern introduced for Doctor Who's sixth recording block, which meant that all of its pre-filming taking place during a dedicated week. Location work occurred at two quarries near Malvern, Worcestershire owned by the Pyx Granite Company, which stood in for various areas of the wasteland outside the Gond city. The first venue, on November 10th, was the Tank Quarry, where model shots were also captured. Cast and crew then moved to the West of England Quarry on the 11th. Four days at the BBC Television Film Studios in Ealing, London followed, from November 12th to 15th. Of particular concern for Maloney's team was material involving the Krotons' hose weapon, the Doctor and Zoe being processed, the Kroton tank, and various effects shots.

Studio work got under way on November 22nd. As usual, all four episodes of The Krotons were recorded on successive Fridays at Lime Grove Studio D in Shepherd's Bush, London. When Episode Three was taped 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.

As had been the case for most of the season, Episode One of The Krotons aired after Grandstand, and before a news update and The New Lucy Show. The following week, on January 4th, 1969, comedienne Lucille Ball replaced herself, with her third sitcom, Here's Lucy, taking the place of The New Lucy Show. Then, as The Krotons concluded on January 18th, a Tom And Jerry cartoon short was added in-between the news and weather and the American import.

• 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.
• Doctor Who: The Complete History #13, 2015, “Story 47: The Krotons”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Second Doctor by David J Howe, Mark Stammers and Stephen James Walker (1997), Virgin Publishing.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.