1199: Returning from the Crusade, Hubert, the new Earl of Mummerset, comes to take possession of Stockbridge Castle, his ancestral home.  The only trouble is, in his absence, demons took possession of his Castle...

1899:  The Stockbridge Mummers’ play takes a wholly unexpected turn, when the dragon slays St George.

These events are not unconnected, the Doctor and Nyssa discover.  There's an alien presence squatting in Stockbridge Castle, and it's their job to expose it. If Turkish knights, killer boars and a gang of rogue paladins don't stop them first…