When the TARDIS lands in England's Middle Ages, the Doctor is captured by a villainous Lord Waldean de Beauvain and imprisoned in his dungeon. The Doctor shares a cell with Haval, once the Sergeant of Arms to the real master of the castle, Lord Geoffrey, who has been replaced by his brother. The Doctor devises a plan and the two escape but are soon imprisoned again, this time by the sorcerer Signus. The Doctor revives the hypnotised Sir Geoffrey, with whom they are imprisoned. Constructing a glider, the Doctor enables Haval to glide from their tower prison and over the castle moat in a bid to seek help from the King's men. In the meantime, the Doctor and Sir Geoffrey must resist an attempt by Signus to take over their bodies by mystical means.