Castle (2009): Season 3, Episode 15

3 - 15: The Final Nail

Castle and Beckett find themselves on opposite sides of the case when Castle's old school friend emerges as the prime suspect in his own wife's murder. As they work to solve the crime, Castle comes to realize that one of two terrible things must be true: Either Detective Beckett is about to arrest an innocent man for murder, or his old friend is a cold-blooded killer.