Letts first work on Doctor Who was as director of the Patrick Troughton serial "The Enemy of the World", and he would direct four serials during his tenure as producer: "Inferno"note , "Terror Of The Autons", "Carnival Of Monsters" and "Planet Of The Spiders". After leaving as producer, Letts returned to direct the Fourth Doctor Serial "The Android Invasion".
Taking over as producer from "Doctor Who and the Silurians", Pertwee's second story as the Doctor, Letts inherited from his predecessor Derrick Sherwin a major change to show's format in which the Doctor was stranded on Earth, exiled by the Time Lords. Although neither Letts nor Dicks was a major fan of this limitation to the show's storytelling possibilities, they largely stuck to it for their first two seasons, before beginning to experiment with returning to space with their third, and completely lifting the exile with their fourth.
Letts also co-wrote four serials during his tenure as producer with his friend Robert Sloman: "The Dæmons", "The Time Monster", "The Green Death" and "Planet Of The Spiders". BBC regulations precluded Letts from being credited as writer, since he was already credited as producer, so "The Dæmons" was credited to the pseudonym "Guy Leopold" and the remaining three gave sole credit to Sloman.
Letts departed as producer, along with Dicks and Pertwee after five seasons in the post, to be succeeded by Philip Hinchcliffe; at the time, he was the longest-serving producer in the show's history. One of Letts' final acts before departing was to cast a little-known, out of work actor called Tom Baker in the role of the Fourth Doctor. It would prove an inspired choice, and it would be seven years and three producers later before another new Doctor needed to be cast. For the last of those years, Letts briefly returned to the show as an executive producer to mentor the last of those producers, John Nathan-Turner, who'd stay in the position all the way until the end of the show's initial run in 1989, surpassing Letts as its longest-tenured producer.
Letts would maintain a close relationship with Doctor Who even after its cancellation and 2005 revival, writing radio plays and novelizations during the '90s and 2000s. He would also be a regular face at conventions and other public events and frequently gave interviews related to the show before passing away at the age of 84 in 2009, having had to cancel a planned con appearance due to health issues related to a lengthy battle with cancer. Said con would add a posthumous dedication to him in their screening of the newly-recolourized third episode of "Planet of the Daleks".
Tropes present in his work:
- Author Appeal: "Planet Of The Spiders", which Letts co-wrote, incorporates his Buddhist beliefs significantly, being set in a Tibetan monastery and depicting regeneration as analogous to the Buddhist take on reincarnation, a take that all later depictions of regeneration would run with. This came back to bite him slightly, as some Buddhists objected to the use of mantras and chants as a means to summon giant evil spiders.
- Chroma Key: Letts was noted as an early pioneer of "Colour Separation Overlay", the forerunner of modern Chroma Key (Green Screen) technology. Its copious use in many stories of the Letts era is very obvious to modern viewers watching the remastered DVD and Blu-ray editions, and can come across as Special Effect Failure to them. However, it should be remembered that it would have been far less obvious and distracting to viewers watching on blurry television sets in the early 1970s.
- Continuity Creep: While Doctor Who never ignored its continuity, under Letts' tenure, the focus grew noticeably stronger, owed in part to the more centralized nature of the UNIT era and the Second Doctor era laying much of the groundwork for story elements that would dominate the Third Doctor's run. The show would dial things back again during most of the Fourth Doctor's run before John Nathan-Turner turned it up again during his tenure as producer.
- Retcon: Letts' tenure as producer marked a gradual change in the way that the Doctor and other Time Lords were depicted. In the first three seasons, they were Ambiguously Human, with the show most often leaning into the idea that Time Lords were an advanced race of humans from the distant future. Under Letts, they explicitly became an alien species, complete with Bizarre Alien Biology courtesy of fellow retcon-friendly Who staffer Robert Holmes. Regeneration in particular would see a big change in approach, going from a presumably artificial process initiated by the TARDIS and/or higher-rankingTime Lords to a biological process that can occur without the need for Gallifreyan technology.