In the season 4 finale, Castle is concerned when a murder case is connected to the conspiracy behind Beckett's mother and is told by the mystery man that Beckett's life is in danger if she investigates. After failing to warn her off subtly, Castle confesses to Beckett how he's been protecting her and she's outraged over his making decisions for her. He confronts her over how he knows she remembers his declaration of love and they both decide they're done. After nearly dying in a confrontation with the killer and quitting the police force, Beckett comes to Castle's apartment, telling him she could only think of him while on the verge of death and then they later kiss passionately.