Earth 1983. The mysterious Black Guardian hovers over the shoulder of English public schoolboy, Turlough, determined to enlist his aid in his deadly scheme - the assassination of the Doctor. Meanwhile the TARDIS is trapped inside a huge space ship in perpetual orbit, a permanent resting place for the miserable Mawdryn and his eight fellow scientists, all condemned to a state of perpetual regeneration. Only the Doctor's life force can free them, but at a deadly cost - the price of his powers of regeneration. With Nyssa and Tegan stranded in the time zone of 1977, and the Black Guardian closing in, the Doctor must face a terrible dilemma...