Merlin - Series 3 - The Changeling

Arthur is pressured into an arranged marriage with a visiting princess, Elena. Merlin's reservations are vindicated when he discovers Elena may not be exactly who she claims to be. Whilst trying to uncover the dark secret hidden inside her, Merlin learns that Elena's nanny is a pixie. But who is she working for? And what do they have to gain from this marriage? Can Merlin get to the bottom of this mystery before Arthur makes the biggest mistake of his life?