The Traitor (1) Poltergeist: The Legacy : Season 4 Episode 6

Alex returns after an unexpected sabbatical as the team turns on Derek when an old case is re-opened. The twenty year old evidence begins pointing towards Derek as a cold blooded killer and old witnesses can no longer rely on their own accounts to save him. As he stands firm in his innocence those closest to him, except for one, lose all confidence in their friend, mentor and Precept.