The Time Lords send the Doctor and Jo to the primitive planet of Peladon. King Peladon and his Chancellor wish Peladon to join the Galactic Federation, and are hosting a delegation from the Federation. Shortly after their arrival, the Chancellor is murdered, apparently by Aggedor - the mythical Royal Beast of Peladon. The Doctor soon discovers that someone is attempting to use the legendary beast to prevent Peladon joining the Federation and trigger a war.