The Black Guardian strikes a deal with a British student named Turlough; 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 the mutated alien scientists while keeping the two Brigadiers from different time zones from meeting?