Mawdryn Undead ( TV story ) 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 . Synopsis . A warp ellipse draws the TARDIS off course . The Fifth Doctor ' s companion s are separated from him not in space , but in time , and he has to deal with a treacherous schoolboy named Turlough . But why does the Doctor ' s old friend , the Brigadier , not remember him at all ? Plot . Part one . 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 materialises on board an apparently empty starliner 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 transmat capsule and encounters the TARDIS crew . The Doctor travels to Earth via transmat , taking Turlough with him , to get rid of the transmat 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 . . . Part two . The interference device explodes , knocking the Doctor backwards and forcing the rock out of Turlough ' s hands . The TARDIS materialises 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 realises 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 transmat 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 starliner . 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 . Part three . Under the pretence 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 transmat 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 stabilise 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 crewmates . 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 . . . Part four . The Doctor explains that he can only regenerate twelve times , and that , as he has already done so four times , helping the mutants die would consume all eight of his remaining regenerations . 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 transmat . Unbeknownst to them , the transmat 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 . Story notes . Production errors . - In the scene in which Turlough crashes the Brigadier ' s car , when he closes in on the other car , the next shot shows the car on the other side of the road , then the next shot shows the car on the right side of the road . - In the same scene , both sides of the road are covered in thick bush , but as the car is about about to crash , an open gate suddenly appears . - While both aspects of the Brigadier are walking the same stretch of corridor in Mawdryn ' s ship , a split - screen effect is used to have them both present , then faded out so that one of them can cross the full length of the set . Unfortunately , the fade is obvious ( One half of the scene clearly shifts in tone ) . Home video and audio releases . DVD releases . This story was released as " Doctor Who : Mawdryn Undead " . Released : Special Features : Notes : Video releases . This story was released on VHS in November 1992 in the UK markets , 1993 in Australian markets and 1994 in US markets .