All 12 episodes of the Black Guardian trilogy from the twentieth season of the long-running sci-fi series. On discovering public schoolboy Turlough lost aboard an apparently abandoned cruiser in space,the Doctor (Peter Davison) transports to Earth in 1983,only to meet up with his old friend friend and UNIT colleague,Brigadier Lethbridge-Stewart,who has apparently lost all memory of him. Back in 1977,Tegan and Nyssa meet the younger Brigadier and a disfigured alien they believe could be a regenerating Doctor. However,it turns out that this is all in fact part of a plot to destroy the Doctor by the Black Guardian,who has made a deal with Turlough to grant him transportation away from Earth if he kills the Doctor. Episodes are: 'Mawdryn Undead (Parts 1-4)','Terminus (Parts 1-4)' and 'Enlightenment (Parts 1-4)'.