Retired from UNIT, Brigadier Lethbridge-Stewart now teaches maths at a boys' boarding school where one of the pupils, Turlough, is secretly an alien. The Black Guardian convinces Turlough to kill the Doctor, and directs him to a concealed transmat capsule. It leads to a spaceship, deserted except for the TARDIS. The Doctor accompanies Turlough back to Earth, and is shocked to discover that the Brigadier has forgotten him. Tegan and Nyssa try to follow in the TARDIS, but find themselves arriving seven years too early. There they meet not only the Brigadier, but also a horribly burned man who claims to be the Doctor.

During the spring of 1981, Doctor Who producer John Nathan-Turner decided to drop Adric from the series, having concluded that he was not succeeding as a companion. In part, he blamed this on the fact that some of his original ideas for the character had not been adequately embraced, such as an untrustworthy streak which was intended to heighten the sense of danger and suspense in Doctor Who. Nathan-Turner still believed that this was a direction worth exploring, albeit via a different character. He was supported by script editor Eric Saward, who was intrigued by the notion of an “evil” companion.

On May 15th, Nathan-Turner issued a character outline for Turlough, a humanoid alien of mysterious origin. Turlough would be the focus of a trilogy of stories at the heart of Doctor Who's twentieth season, continuing a pattern of short, linked arcs which the show had employed over the previous two years. This time, the connective tissue would be provided by the machinations of the Black Guardian, a cosmic entity who had been introduced during the Key To Time storyline that ran through Doctor Who's 1978-79 season. Turlough would be revealed as an agent of the Black Guardian, secretly scheming to kill the Doctor.

John Nathan-Turner envisaged the trilogy as slowly peeling back the layers of Turlough's duplicity

Nathan-Turner envisaged the new trilogy as slowly peeling back the layers of Turlough's duplicity. In his introductory adventure, Turlough would arrange to have the Doctor save his life, and thereby join the TARDIS crew. He would then covertly act against the Doctor in the trilogy's middle installment. The final story would see the truth about Turlough exposed and the defeat of the Black Guardian, whereupon Turlough could either be retained as a companion, or written out of Doctor Who if the character was not working well.

It was decided that Turlough would be introduced in the third serial of Season Twenty. Originally, this was intended to be “The Song Of The Space Whale” by Pat Mills and John Wagner, who wrote the regular comic strip for the Doctor Who Weekly periodical. However, Wagner dropped out of the project in late 1981, and the relationship between Mills and Saward became uneasy. By May 1982, it was clear that “The Song Of The Space Whale” would not be ready to enter production at the end of August.

At first, it was thought that Enlightenment -- the story intended to conclude the trilogy -- might be brought forward in the production order to take the place of “The Song Of The Space Whale”. However, Saward had already been discussing ideas for Doctor Who with Peter Grimwade, a director who had also written Season Nineteen's Time-Flight. Grimwade was interested in developing a Doctor Who version of the tale of the Flying Dutchman -- a mariner's legend with its origins in the eighteenth century, about a ghost ship and the sailors aboard who were doomed to plough the seas forever. This suggested a narrative about a spaceship which travelled ceaselessly through the stars, carrying a crew of immortals.

Grimwade also wanted to write about a series of interrelated events taking place in the same location, but in different time periods. His inclination was to separate these settings by several centuries, but he was encouraged to bring them closer together so that one character could be present in both time periods. Ian Levine, who was acting as the Doctor Who production office's unofficial continuity adviser, suggested having one of the Doctor's former companions fulfil this function. Grimwade agreed, and thought of science teacher Ian Chesterton, one of the programme's original protagonists. This, in turn, inspired the setting: an English boys' school, which would allow Grimwade to draw upon his own unhappy experiences at such an establishment. The title became Mawdryn Undead, derived from the Welsh words marw (“dead”) and dyn (“man”).

When The Song of The Space Whale was dropped, Peter Grimwade was asked to hastily develop Mawdryn Undead to replace it

Early in its development, it was thought that Mawdryn Undead might be broadcast towards the end of Season Twenty. However, when “The Song Of The Space Whale” was dropped from the schedule, Grimwade was asked to hastily develop his scenario to replace it. Since it would now serve to introduce Turlough -- rather than include him as an established character -- Grimwade merged his role with that of one of the schoolboys. A storyline was commissioned on May 27th; Enlightenment was restored to its original place in the production order, with Mawdryn Undead now positioned as the third serial in both the recording and broadcast schedules. By now, Grimwade and Saward had settled on 1983 and 1977 as the serial's two time periods, the latter offering Queen Elizabeth's Silver Jubilee as a backdrop for the story's events.

Unfortunately, it was soon discovered that stage commitments would prevent William Russell, who had played Ian Chesterton, from appearing in Mawdryn Undead. The production team then unsuccessfully approached Ian Marter about reprising his role as Harry Sullivan, the surgeon-lieutenant who had accompanied Tom Baker's Fourth Doctor in 1975. Finally, Nathan-Turner turned to Nicholas Courtney. He had played Brigadier Lethbridge-Stewart of the United Nations Intelligence Taskforce (UNIT) on a recurring basis from 1968 to 1975, latterly in Terror Of The Zygons. Courtney had attended Baker's farewell party in early 1981 and, at that time, he had indicated his willingness to return to Doctor Who. Courtney was available during the recording dates for Mawdryn Undead and so he was contracted for the serial on June 4th, the same day that Grimwade's scripts were commissioned.

Although it was not a natural fit for the character, Grimwade inserted the Brigadier into his narrative, with the idea that he was now retired from UNIT and teaching maths. Levine pointed out that setting part of the story in 1977 would break with the continuity of Lethbridge-Stewart's original serials, which had been intended to take place a few years ahead of their broadcast dates. As such, it would be inconsistent to depict the Brigadier as being retired in 1977 when stories such as Pyramids Of Mars had indicated that he was still active with UNIT into the Eighties. Nathan-Turner liked the connection with the Silver Jubilee, however, and did not act on Levine's advice... in the process giving rise to perhaps the most notorious contradiction in the chronology of Doctor Who.

In writing the scripts for Mawdryn Undead, Grimwade wanted to suggest that part of the reason the Doctor accepted Turlough so readily was because he still mourned the loss of Adric; however, this motivation was eventually dropped. Grimwade also specified that a flashback sequence should be included in the Episode Two scene where the Doctor helped the Brigadier regain his memory. Similar moments had met with some acclaim in two recent serials he had directed: Season Eighteen's Logopolis and Season Nineteen's Earthshock. It also accorded with Nathan-Turner's desire to appeal to the programme's fanbase.

Valentine Dyall agreed to reprise the Black Guardian, despite being in ill health

Assigned to direct Mawdryn Undead was Peter Moffatt, whose most recent Doctor Who work had been on The Visitation a year earlier. Prominent in Moffatt's cast was Valentine Dyall, playing the Black Guardian. Dyall had originated the role in 1978's The Armageddon Factor and agreed to reprise it for the new trilogy, despite being in ill health and losing confidence in his abilities as a performer.

The other key casting decision for Mawdryn Undead was the role of Turlough. This proved to be an arduous process, and Nathan-Turner began to grow concerned that no suitable actor would be found. Finally his partner, BBC production manager Gary Downie, recommended Mark Strickson, who had recently appeared in episodes of the medical drama Angels. Strickson auditioned for Turlough, and Nathan-Turner found himself concurring with Downie's assessment.

Before he could offer Strickson a contract to play Turlough, however, Nathan-Turner was informed that the production team of Angels was planning to add Strickson to their regular cast. Although the actor had not formally accepted this promotion, the Angels writers were already adjusting their scripts accordingly. Strickson, however, was irritated by this act of presumption, and decided to join Doctor Who after all. On June 14th, he was contracted for eighteen episodes, encompassing the remainder of Season Twenty.

Although Nathan-Turner's notes had described Turlough as being blond like Strickson, concerns were now raised that he resembled Peter Davison too closely. As such, Nathan-Turner suggested that Strickson should shave his head to play Turlough. The actor was amenable, but only if he were paid an extra fee to compensate for any resulting loss of work in the weeks after he left Doctor Who. Nathan-Turner balked at this request, and make-up designer Sheelagh Wells instead found a way to efficiently dye Strickson's hair ginger. He was also given a shorter trim than Davison to further differentiate them. It was Strickson's idea to dress Turlough in dark, sombre clothes, which he felt would make an effective contrast with the Doctor, Nyssa and Tegan.

Mawdryn Undead was the first serial to go before the cameras after an almost three-month break in the production of Doctor Who, during which Davison recorded the final year of his sitcom Sink Or Swim. Work began with location filming at the Trent Park campus of Middlesex Polytechnic in Cockfosters, London. Scenes in and around Brendon School were recorded there between August 24th and 27th. Courtney was made up slightly differently to play the two versions of the Brigadier. The 1977 model retained the character's familiar moustache, while the 1983 version was clean-shaven; the latter also had greyer hair, including a bald spot.

Studio recording began with a two-day session on September 8th and 9th at BBC Television Centre Studio 6 in White City, London. The sets for the TARDIS console room, the school corridor and sick bay, and the transmat capsule were in use on both days. Material in the Brigadier's residence was also taped on the 9th. On September 15th, a photocall was held to introduce Strickson as Turlough, and to bid farewell to Sarah Sutton; her last story as Nyssa would be Terminus, the next serial in production.

The second studio block then spanned September 22nd to 24th, with Grimwade's team shifting to TC8. The first order of business was a remount of the TARDIS console room scenes involving Mawdryn. Wells had been disappointed with the way Mawdryn's brain protuberance blended with his skin and so, during the intervening fortnight, costume designer Amy Roberts hid the join by adding a metal headband to the mutants' outfits. The rest of the block largely focussed on material aboard the spaceship. In addition, the remaining sequences in the transmat capsule were recorded on the middle day, and those in the Black Guardian's void on the final day.

The flashback sequence in Episode Two brought together clips from many of Courtney's previous Doctor Who serials. The images selected included a younger Brigadier (from The Three Doctors), a Yeti (The Web Of Fear), a Cyberman (The Invasion), the Second Doctor (The Three Doctors), an Axon (The Claws Of Axos), a Dalek (Day Of The Daleks), the Third Doctor (Spearhead From Space), the First Doctor (The Three Doctors), the K1 Robot (Robot), a Zygon (Terror Of The Zygons), and the Fourth Doctor (Terror Of The Zygons).

• Doctor Who Magazine #223, 17th January 1996, “Archive: Mawdryn Undead” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #1, 2001, “Diamond Life” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #36, 2017, “Story 125: Mawdryn Undead”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Handbook: The Fifth Doctor by David J Howe and Stephen James Walker (1995), Virgin Publishing.
• In·Vision #65, June 1996, “Production” edited by Anthony Brown, Cybermark Services.