Merlin: Season 3, Episode 6

3 - 6: The Changeling

Princess Elena arrives in Camelot to visit Uther. Uther arranges a marriage between Arthur and Elena, but her nanny Grunhilda is working with the Sidhe Elder, who plans to use this marriage to take over Camelot. Will Merlin be able to stop their wicked plan or will Arthur make the biggest mistake of his life?