Whilst out on a routine hunting trip, Arthur and his men encounter a huge and terrifying monster - the Questing Beast. Uther ignores Gaius' warning, that the creature's bite is always fatal, and sends Arthur and his knights out to kill it. The beast knocks Arthur unconscious and bites his shoulder. As Arthur's life hangs in the balance, Merlin must travel to the Isle of the Blessed and there strike a potentially fatal bargain with the sorceress, Nimueh. The young warlock is the only one who can save Arthur - but at what cost?