The Fourth Doctor and Sarah Jane are joined in the TARDIS by UNIT's Medical Officer, Surg. Lieut. Harry Sullivan. Despite impressive credentials in science, Sullivan accidentally changes the ship's course and they land on a seemingly abandoned space station. It's classic 70s sci-fi -- Earth is uninhabitable. Samples of all organic life were sent into stasis in space until the Earth is safe for human life. But something got into the wiring . . . The Ark, and the Earth, need the Doctor, Sarah Jane and Harry to save it.