England, March 1215. King John is visiting the castle of Sir Ranulph Fitzwilliam. The arrival of the TARDIS disturbs a medieval joust, but the Doctor and his companions are proclaimed to be friendly demons by the King, who seems strangely interested in their "blue engine." It soon becomes clear that neither King John or his Champion, Sir Giles Estram, are who they pretend to be. One of the Doctor's oldest and deadliest enemies threatens the future of democracy on Earth, and he must be stopped!