The first part of the Black Guardian trilogy. On discovering public schoolboy Turlough lost aboard an apparently abandoned cruiser in space, the Doctor transmats 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. This is all in fact part of a plot to destroy the Doctor by the Black Guardian.