In England in 1983, the Black Guardian strikes a deal with a young student named Turlough, who is secretly from another planet but stranded on Earth. He will return him to his home planet if Turlough will kill the Doctor for him. Meanwhile the Doctor, Tegan and Nyssa are near Turlough's school, trying to repair a hidden device that has the TARDIS trapped in perpetual orbit, when they run into an old friend of the Doctor's... twice. Can the Doctor avoid the machinations of both Turlough and a ship of mutated alien scientists while keeping two versions of his old friend The Brigadier from different time zones from meeting?
—Jay H