Still trapped in E-Space, the Doctor's TARDIS materialises on a medieval planet. The townsfolk live in fear of the Three Who Rule, who govern from their mighty castle. Investigating, the Fourth Doctor discovers that the Three Who Rule are ancient astronauts who became vampires long ago, and their castle is actually their spaceship. When Romana and Adric are kidnapped, the Doctor must ally himself with a band of renegade peasants to stop the resurrection of one of Gallifrey's greatest enemies: the King Vampire itself.