As The Doctor attempts to save his own life from a small group of undying mutants ("Mawdryn Undead"), and then the lives of those suffering from a leprosy-like disease ("Terminus"), Turlough is introduced as the Black Guardian's sort-of-unwilling lackey / assassin, a retired Brigadier Alistair Gordon Lethbridge-Stewart makes his return, and Nyssa bids the TARDIS and her friends farewell.