When the Doctor sees an archeological dig on the television he races with Jo to try to stop it.  The Doctor is knocked out cold and Benton and Yates arrive on the scene just before a heat barrier cuts the village off.  The Master summons Azal, the last of a race called Dæmons, whose spaceship is buried within the barrow.  Azal is to appear three times.  He is to decide to transfer his powers to another or to destroy the planet.  The Master want the powers but Azal offers them to the Doctor.  The Doctor refuses.  Azal decides to kill the Doctor but Jo demand that he kill her instead.  The Dæmon self-destructs because of Jo's apparent self sacrifice.  The Master taken to await trial for his crimes.