During two closely related time zones, the Black Guardian recruits schoolboy Turlough to kill the Doctor in exchange for a return to his home planet. Then, the Doctor meets Turlough and goes to the present to fix the transmat equipment on Earth. He also plans for Nyssa and Tegan to follow him on the TARDIS, but they get diverted to the past. William Hartnell, Patrick Troughton and Jon Pertwee star.