Morgana takes the dying Morgause to the Isle of the Blessed where she unwittingly tears the veil to the spirit world, unleashing the Dorocha, invisible ghosts that begin to attack the kingdom. At Camelot, where a weakened Uther has given over much of his rule to his scheming brother Agravaine, Merlin senses what is happening but finds his magic inadequate to stop it. As Camelot fills with refugees, Gaius reasons that, as one death opened the portal, only another will close it. Arthur offers himself as the sacrifice and travels to the Isle of the Blessed with Merlin and a group of young knights. Here, after Arthur has finally praised him for his bravery, Merlin appears to have a fatal encounter with one of the Dorocha.

When Merlin falls into Morgana's hands, he becomes a deadly weapon in her fight for supremacy. Using ancient magic, she pits friend against friend to create the perfect assassin. An oblivious Arthur is in great danger. Will anyone notice Merlin's unusual behaviour before he does the king some serious harm?

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.