The Doctor and UNIT are called upon to investigate strange events surrounding Dr. Tyler's research into cosmic rays. UNIT HQ comes under attack by an alien force, and the Doctor has no other option but to call on the Time Lords for help. The Time Lords, too, are under attack - from a mysterious power emanating from a black hole in space. With power reserves falling, the only way they can help the Doctor is to break the First Law of Time and allow him to help himself. The First and Second Doctors are lifted out of their time streams and sent to Earth to help the Third. Between the three of them, they must find a solution to the threat that not only endangers UNIT and Earth, but also the future of the Time Lords themselves.