Arthur has been king for three years without incident, but now an expedition north to the land of Ismere, led by Gwaine and Percival, is captured by Morgana and fellow sorcerer Ruadan and set to work in the mines to locate a mystical key. A search party headed by Arthur and Merlin goes after them, coming upon a village whose inhabitants have been massacred, A dying Druid warns Merlin of Arthur's bane - a battle where the young king will be slain. Ruadan's daughter Sefa is a spy working as Guinevere's maid, who warns her father of Arthur's army heading for Ismere through the lands of his ally Queen Annis. The group is thereby ambushed and Merlin and Arthur captured by Mordred, whom Merlin recognizes from the prophecy as being Arthur's killer.

After Arthur is unseated from his horse, his saddle is shown to have been tampered with and stable groom Tyr Seward blamed. Imprisoned, he tells Merlin he saw the true culprit but was told his mother would die if he informed on them. The actual saboteur, Guinevere (enchanted by Morgana) kills Tyr, and Arthur realises too late his innocence. Guinevere then poisons Arthur and casts the blame on Merlin, leaving an imprisoned Merlin to use magic to save himself and the king. To cover herself, Guinevere summons the apothecary from whom Morgana purchased the poison to pass subsequent blame onto Morgana as the would-be assassin, though Merlin knows better.