Merlin - Season 3, Episode 08: The Eye of the Phoenix
Trailer
Merlin is the first to discover Morgana's changed loyalties but he must tread carefully as Uther's affection for his 'ward' has increased with her one-year absence. Arthur is likewise blind to Morgana's treachery, forcing Merlin and Gaius to be even more wary of detection as they thwart her schemes.