Serial: 6F  Episode Nos. 594, 595, 596, and 597.  Title: Mawdryn Undead

From a spaceship trapped in perpetual orbit landed a degenerating creature called Mawdryn on Earth via a transmat capsule in the year 1977.  There, he was mistaken for the Doctor by Tegan, Nyssa, and the retired Brigadier Lethbridge-Stewart.  Meanwhile, the Doctor is trapped without the TARDIS in the year 1983 with only an older Brigadier who strangely
doesn't remember him and an odd young man named Turlough to help him.  What the Doctor doesn't realize
until it's too late is that circumstances have been arranged against him by the Black Guardian, with Turlough acting as his
agent....

This story marks the debut of Mark Strickson as Turlough.

starring Peter Davison as the Doctor, Sarah Sutton as Nyssa, Janet Fielding as Tegan, Nicholas Courtney as Brigadier Lethbridge-Stewart, Mark Strickson as Turlough, and Valentine Dyall as the Black Guardian. Written by Peter Grimwade, Produced by John Nathan-Turner, and Directed by Peter Moffatt.

Originally transmitted from 1 February 1983 to 9 February 1983 on BBC1.