The Master, posing as Professor Thascales, obtains the Crystal of Kronos, a relic of ancient Atlantis. Using the Crystal, the evil Time Lord summons Kronos, a powerful Chronovore native to the time vortex. The Doctor pursues his enemy back in time to Atlantis, where he has one last chance to stop the Master from gaining permanent control over Kronos, and unleashing the unstoppable force onto the world.

After cowriting The Daemons with producer Barry Letts for Doctor Who's eighth season (under the joint pseudonym Guy Leopold), Robert Sloman was asked to develop an adventure which would feature the Doctor combatting his old enemies, the Daleks, for the first time since Season Four's The Evil Of The Daleks. Sloman was commissioned to write a storyline for “The Daleks In London” on May 25th, 1971. Not long thereafter, however, Letts and script editor Terrance Dicks decided that the Daleks would be better utilised in the serial planned to start off Doctor Who's ninth season, Louis Marks' “Years Of Doom”. Marks' adventure was rewritten to incorporate the Daleks, becoming Day Of The Daleks, while “The Daleks In London” was scrapped.

Letts still wanted Sloman to contribute a serial to Season Nine, however, and asked the writer to develop a new storyline under his original commission. This time, Letts was interested in a story more akin to The Daemons, featuring a mix of science-fiction and mythology. Other elements requested of Sloman were an historical setting (suggested by the fledgling Doctor Who Fan Club, as it was something not attempted in the series since The Evil Of The Daleks), UNIT and the Master.

Sloman set to work on an idea called The Time Monster, whose inspiration came during a walk outdoors when Sloman heard an airplane flying overhead and was reminded of the sound of German bombers from World War I. From this experience came the idea of “time slippages”. The six scripts for The Time Monster were commissioned on December 28th; this story, designated Serial OOO, would be the last of the new season. (Unlike Jon Pertwee's first two years, when an assignment of twenty-five episodes meant that the production team had to commission at least one serial with a nonstandard number of episodes, Season Nine had been expanded to twenty-six installments so that only four- and six-part stories were needed.)

Whereas The Daemons had taken much of its imagery from medieval legends about the Devil, for The Time Monster Sloman turned his attention to more ancient times and the mythology of the Greeks. The name Kronos was a variant of Cronus, the chief god amongst the Titans, an elder group of deities said to be the progenitors of the more familiar Olympian gods; Cronus was subsequently identified by the Romans with their god Saturn. Sloman also used the Greek word thascales (or thascalos), meaning “master”, as the evil Time Lord's new alias.

Letts continued to have significant input into Sloman's scripting, seeing The Time Monster as an opportunity to delve into the Doctor's background and motivations (via his tale about the hermit on his home planet). In Buddhist terms, Letts wanted to portray the Doctor as someone merely “semi-enlightened”: able to see the universe more clearly than most, but still possessing personal flaws.

Paul Bernard made his second directorial turn of the season on The Time Monster, having earlier worked on Day Of The Daleks. This was also the first time since the Dalek story that the three UNIT regulars returned to Doctor Who. Their contracts were issued during March 1972: Nicholas Courtney's on the 13th, John Levene's on the 22nd, and Richard Franklin's on the 31st.

Production of The Time Monster started with two days at the Ealing Television Film Studios on March 29th and 30th; filming concentrated on scenes such as those involving the Minotaur (portrayed by David Prowse, who would go on to play Darth Vader in the first three Star Wars movies) in the labyrinth, and special effects shots featuring Kronos. Cast and crew then headed out on location, beginning at Swallowfield Park in Swallowfield, Berkshire on April 4th and 5th for scenes at the Newton Institute.

On the 6th, road scenes were filmed at two Hampshire locales -- Stratfield Saye Park in Stratfield Saye and School Lane at Heckfield Heath. Stuntman Greg Powell, who played the knight attacking the UNIT vehicles, was hurt at the first of these locations. He was thrown from his horse while performing the stunt when the animal did not follow the planned route and ended up running into a lorry. Accounts vary as to the extent of the injuries to both Powell and his mount. Material in the woods was also captured on April 6th at Mortimer Lane in Mortimer, Berkshire. Remaining road sequences were completed on the 7th, at Stratfield Saye and Old Church Farm in Hartley Witney, also in Hampshire. Jon Pertwee and Katy Manning became lost while filming scenes in Bessie using a side-mounted camera, forcing Bernard to send out search parties to track them down.

Studio recording for the entirety of Season Nine was organised into biweekly two-day sessions. Unlike previous serials, whose recording days had been Mondays and Tuesdays, however, The Time Monster was taped on Tuesdays and Wednesdays, beginning with episodes one and two on April 25th and 26th in BBC Television Centre Studio 3.

Parts three and four were recorded two weeks later, on May 9th and 10th in TC4. This block saw the introduction of the new TARDIS set (used for both the Doctor's and the Master's TARDISes). Designed by Tim Gleeson, it failed to impress Letts, who felt that the larger and more stylised roundels Gleeson had used looked like washing-up bowls. It turned out that Letts' objections were academic, as the set was damaged in storage prior to the tenth production block, and so would only be seen in The Time Monster. Also taped on the 9th was the last scene of episode six, the only material from the final two installments which involved Courtney, Levene, Ian Collier or Wanda Moore, or the laboratory set. As it transpired, this would therefore be the last time that Courtney and Levene worked alongside Roger Delgado in Doctor Who.

Taping resumed with the final studio session on May 23rd and 24th, back in TC3. By now it had been realised that episode four was underlength, and so additional material was recorded on the 23rd to fill out the running time. Most of part five was then taped, with the exception of the final five or so minutes, which were enacted alongside episode six on the 24th.

Although this meant that Season Nine was now complete, the ninth production block was as yet unfinished. For the first time since the Sixties, it had been decided to tape a story for the next season prior to the summer break, and so the recording block continued with Carnival Of Monsters. While this was in the studio, The Time Monster part six aired on June 24th, drawing Doctor Who's ninth season to a close.

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• Doctor Who Magazine #268, 26th August 1998, “Archive: The Time Monster” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Family Affair” by Andrew Pixley, Panini Publishing Ltd.