Mawdryn Undead is the third serial of the 20th season in the British science fiction television series Doctor Who, which was originally broadcast in four twice weekly parts from 1 February to 9 February 1983. The serial was the first of three loosely connected serials known as the Black Guardian Trilogy, and introduced Mark Strickson as a new companion, Vislor Turlough, as well as reintroducing Nicholas Courtney as Brigadier Lethbridge-Stewart. The character had not been seen on the series since the Fourth Doctor serial Terror of the Zygons almost eight years earlier.

In 1983, Brigadier Lethbridge-Stewart has retired from UNIT, and teaches mathematics at Brendon Public School. One of his students, Turlough, a stranded alien Trion posing as a human, takes the Brigadier's classic car for a joyride but ends up in a crash. While unconscious, Turlough is contacted by the Black Guardian, who seeks to kill the Doctor for his interference in acquiring The Key to Time. The Black Guardian offers Turlough passage off Earth if he kills the Doctor, to which Turlough agrees, and is given a communication device through which the Black Guardian gives him orders.

The Doctor, Tegan and Nyssa, aboard the TARDIS, find themselves caught in the warp ellipse of a starliner that is trapped in time. Materialising aboard the ship, they find a transmat capsule as the source of interference that is trapping the TARDIS, and are soon joined by Turlough, following the Black Guardian's orders. The Doctor takes Turlough with him to Earth to locate the other end of the transmat, leaving Tegan and Nyssa aboard the TARDIS on the ship. After the Doctor corrects the transmat device on Earth, the TARDIS attempts to materialise but soon vanishes. Without any idea where to look, the Doctor talks to the Brigadier at the school, surprised that the Brigadier has forgotten their past histories due to some type of trauma. However, on mention of finding Tegan and the TARDIS, the Brigadier recalls seeing the TARDIS in 1977. The Doctor attempts to coax the details of these events from the Brigadier.

...
Wikipedia