Mawdryn Undead was the third story of Doctor Who’s twentieth season. It saw the return of the Brigadier, and was the first story in the “Black Guardian trilogy”. It also marked the first appearance of Vislor Turlough as both a companion and a villain.
Due to its use of very specific date, such as placing the Brigadier’s retirement during 1976, this story attempted to address the so-called UNIT dating controversy. However, in so doing, it became a major part of that very controversy.
In 1983, the former UNIT brigadier Alistair Gordon Lethbridge-Stewart teaches mathematics at Brendon Public School. A mischievous student, Turlough, convinces his classmate Ibbotson to join him in a joyride in the Brigadier’s priceless antique car, which they accidentally veer off the road and crash. While unconscious, Turlough is contacted by the sinister Black Guardian. The Black Guardian knows Turlough is actually from another planet. He offers Turlough transportation off Earth if he will kill the Guardian’s greatest enemy: the Doctor.
At the same time, the Fifth Doctor, Tegan and Nyssa have problems of their own. The Doctor’s TARDIS is caught in a warp ellipse and materializes on board an apparently empty star-liner locked in a perpetual orbit in time and space. Turlough, on the Black Guardian’s instructions, transports himself onto the liner by means of a trans-mat capsule and encounters the TARDIS crew. The Doctor travels to Earth via trans-mat, taking Turlough with him, to get rid of the trans-mat interference trapping the TARDIS on the liner. The source of the interference is not far from the school grounds. As the Doctor is sorting out the device, Turlough, responding to the Black Guardian’s exhortations, picks up a large rock and prepares to smash it on the back of the Doctor’s head…
The interference device explodes, knocking the Doctor backwards and forcing the rock out of Turlough’s hands. The TARDIS materializes nearby, but vanishes shortly after, seemingly leaving the Doctor stranded. The Doctor meets the Brigadier at the Brendon school, but is puzzled when his old comrade-in-arms does not remember him. He eventually works around the apparent gap in the Brigadier’s memory by mentioning Jo Grant, the Yeti and other past adventures. To the Doctor’s surprise, the Brigadier claims to remember Tegan, having met her shortly before his nervous breakdown in 1977. The Doctor realizes that the TARDIS hasn’t disappeared; the destruction of the device knocked the TARDIS off a time track and sent it back six years.
In 1977, Tegan and Nyssa encounter the trans-mat capsule. Inside is an alien-looking humanoid whom they initially believe is the Doctor, horribly injured. Meeting the younger Brigadier, they bring the alien and him back to the star-liner. Tegan, Nyssa and the Brigadier enter the TARDIS control room. The alien, who is a scientist called Mawdryn, now wearing the Doctor’s old coat, turns to face them. The top of his skull is missing, revealing his pulsing brain. Nyssa screams in horror.
Under the pretense of being the Doctor, suffering a failed regeneration, Mawdryn convinces the others to bring the TARDIS to the ship, claiming it has restorative capabilities. Meanwhile, the Doctor, Turlough and the present-day brigadier use a TARDIS homing device on the damaged trans-mat to bring themselves to the ship, splitting up soon after to search for Tegan and Nyssa. Under the Black Guardian’s instruction, Turlough opens a secret room, revealing several humanoids suffering from the same malady as Mawdryn. Mawdryn fools the present-day Brigadier into using a stolen Time Lord device to stabilize his condition. Finding the TARDIS, the Doctor reunites with Tegan and Nyssa. When he learns there are two Brigadiers aboard, he has to try to keep them apart lest the resulting energy discharge prove catastrophic.
The Doctor and Nyssa meet with the present-day Brigadier and Mawdryn. Mawdryn reveals his fellow scientists and he tried to steal the secret of regeneration from the Time Lords, but caused perpetual mutations, leaving them practically immortal but in horrible agony. Tegan is chased into the room by Mawdryn’s mutated crew mates. The mutants take their places in the regeneration room and Mawdryn pleads with the Doctor to help them die by giving them his energy. The Doctor refuses, explaining to Tegan that if he did so it would mean the end of him as a Time Lord…
The Doctor explains that, since each mutant would consume one of his regenerations, helping them all to die would rob him of the eight he has left. Although Mawdryn proclaims that the only thing they desire is their deaths rather than the Doctor’s ‘murder’, the Doctor refuses, unwilling to sacrifice his remaining lives to save them from their own mistake.
Under the Black Guardian’s orders, Turlough locates the Brigadier of 1977 and locks him in a side room to keep the two Brigadiers from touching.
Trying to leave in the TARDIS, the Doctor discovers that Tegan and Nyssa have been infected by the same malady as Mawdryn and his compatriots. Attempted time travel accelerates their conditions and reversing out of the warp ellipse only regresses Tegan and Nyssa into children. The only cure, it seems, is to do what Mawdryn demands; the Doctor must give up his remaining regenerations. Meanwhile, the past Brigadier escapes the room and encounters Mawdryn and his crew. Fearing the time imbalance, they send him into the trans-mat. Unbeknownst to them, the trans-mat cannot make the trip while the TARDIS is on the vessel and the capsule returns to its station seconds later.
Hooking himself up to Mawdryn’s apparatus, the Doctor is about to sacrifice himself when the two Brigadiers meet and touch hands, causing a discharge of temporal energy at precisely the right instant to act as a substitute power source. Tegan and Nyssa are cured, the alien scientists end their undead existence and the Doctor remains a Time Lord. The younger Brigadier, however, will not remember his time with the Doctor until they meet again in 1983. The Doctor, Nyssa and Tegan deliver the 1977 and 1983 versions of the Brigadier back to their respective times. Turlough is in the TARDIS control room when they return. He asks if he can join them and the Doctor comments that he already has. In space, Mawdryn’s ship self-destructs.