Series 20: Mawdryn Undead

In 1983, the former Brigadier Lethbridge-Stewart teaches mathematics at Brendon Public School, where Turlough is a student. In the aftermath of a car accident in which Turlough and another student take the Brigadier's classic car for a joyride and crash, Turlough is contacted by the sinister Black Guardian, whom the Doctor thwarted during the quest for the Key to Time. Seeking revenge, the Black Guardian offers Turlough transportation off Earth if he will kill the Doctor.

Meanwhile, the Fifth Doctor, Tegan and Nyssa have problems of their own. The TARDIS is caught in a warp ellipse and materialises on board a starliner locked in a perpetual orbit in time and space. Turlough, under the Black Guardian's instructions, transports himself onto the liner from Earth 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 that is trapping the TARDIS on the liner. Unfortunately, when the TARDIS tries to materialize on Earth, it vanishes. The Doctor meets the Brigadier at the Brendon school, but is puzzled when his old comrade-in-arms does not remember their time together at first. When the Doctor says he has to find Tegan and his TARDIS, the Brigadier remembers meeting her in 1977. The Doctor realizes that the TARDIS is right there - just six years earlier - and tries to get the Brigadier to remember the events that led to his nervous breakdown in 1977.

In 1977, Tegan and Nyssa encounter the transmat capsule, but inside is an alien-looking humanoid whom they initially believe is the Doctor, horribly injured. Meeting the younger Brigadier, they bring him and the alien back to the starliner, which is actually the prison of a group of alien scientists who had been trying to discover the Time Lord secret of regeneration. As Mawdryn, the leader of the group explains, they only succeeded in trapping themselves in a cycle of perpetual mutation and regeneration and now long for death. When the Doctor finds out that there are two Brigadiers aboard, he has to try to keep the two apart lest the resulting energy discharge prove catastrophic.

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. The only cure, it seems, is to do what Mawdryn demands: the Doctor must give up the energy from his remaining regenerations. 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. Tegan and Nyssa are cured, the alien scientists succeed in ending 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...

After returning the Brigadiers to their respective time zones, Turlough asks if he can join the Doctor in his travels. The Doctor agrees, apparently not realizing he is taking an assassin into the fold.

Cast
Doctor Peter Davison (Fifth Doctor)
Companions Sarah Sutton (Nyssa)
Janet Fielding (Tegan Jovanka)
Mark Strickson (Vislor Turlough)
Guest stars
Nicholas Courtney — The Brigadier
David Collings — Mawdryn
Valentine Dyall — Black Guardian
Angus MacKay — Headmaster
Stephen Garlick — Ibbotson
Roger Hammond — Dr Runciman
Sheila Gill — Matron
Peter Walmsley, Brian Darnley — Mutants

Production
Writer Peter Grimwade
Director Peter Moffatt
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6F
Series Season 20
Length 4 episodes, 25 minutes each
Originally broadcast February 1–February 9, 1983

source: wikipedia