The Doctor's TARDIS is stolen, and tracking it down reveals the existence of an alien named Kalid controlling events in the past. While the Doctor challenges Kalid's motives for being there, Tegan and Nyssa discover the source of Kalid's power - an even greater threat for the Doctor.
Quest roles:
Anthony Ainley(The Master), Matthew Waterhouse(Adric), Keith Drinkel(Flight Engineer Scobie), Michael Cashman(First Officer Bilton), Richard Easton(Captain Stapley), Nigel Stock(Professor Hayter)