While investigating the shooting death of her ex-partner in New York, Beckett comes to believe the suspect has fled to Los Angeles. Against orders, she travels to L.A., going renegade to track down the killer, with Castle unexpectedly along for the ride. As they dig deeper into the motives, they uncover a trail that leads from a rooftop pool in Hollywood to the sandy beach of Santa Monica. Along the way, they visit the set of the "Heat Wave" movie and enlist the cast to help break the case. But as they get closer to apprehending the killer, Castle begins to wonder whether Beckett is out for justice -- or revenge.

Kate Beckett's investigation into her mother's death is reactivated when Hal Lockwood - a professional hitman with knowledge of Beckett's mother's murder - stages a spectacular escape during a court hearing. As police launch a manhunt for Lockwood, Castle and the team uncover evidence of a deeper conspiracy involving a mysterious figure with law enforcement connections who is linked to both Lockwood's escape and the killing of Johanna Beckett. The search for Lockwood and the quest to identify his co-conspirator leads to a shocking and deadly conclusion.