When Castle and Beckett are handed the murder of a couples therapist and Ryan and Esposito are assigned to investigate the death of a dead man found shot in a park, Castle makes a bet on which pair can solve their murder first. But when both investigations hit dead ends and the victims are found to be strangely connected, the two teams work together to solve the peculiar puzzle.
- The Bet: Castle, Esposito and Ryan bet on who solves their case first.
- Contrived Coincidence: Both victims, aside from being linked in a "Strangers on a Train"-Plot Murder case, were also involved in bringing charges against a domestic abuser—a Red Herring who the real murderers are completely unaware of.Castle: It so happens that our two victims were connected. It just had nothing to do with their deaths.
- Did I Say That Out Loud:Ryan: So much for my famous warm honey-milk with Jenny tonight. Did I just say that out loud?
- Distracted by the Sexy: Laney shows up to the crime scene in a sexy dress. Castle can't help but stare.Laney: Quit looking at the girls, Castle.
Castle: [jerks gaze away] Roger that. - "Eureka!" Moment: One of the more prominent ones; frustrated at the lack of progress on either case, Beckett suggests to Ryan and Esposito, "You take our case, we'll take yours." Castle suddenly says, "You take mine, I'll take yours... could it be that simple?"
- Evasive Fight-Thread Episode: The initial competition, over which team can solve their case first, ends in a tie when a connection between both victims reveals it's actually a single case. Then they bring the two suspected conspirators in for questioning, and decide that the bet is back on, as they race to see who can get a confession from their suspect first. In the end, Beckett and Castle get the confession first—but from Ryan and Esposito's suspect, not their own. The final scene is all four of them arguing over who really won.
- Henpecked Husband: The murderer hated this and had his wife killed.
- Not So Above It All: Beckett lectures the others for betting on who will close their case first—then a promising piece of evidence drops into her lap, and she immediately joins in the bet, placing $100 that she and Castle will win.
- Pass the Popcorn: The opening scene shows Castle eating popcorn while watching the police struggling with colorful prisoners in the booking office. He comments that they should sell tickets.
- Shown Their Work: The Hudson is a tidal estuary, where you get brackish water as the river enters the bay. Thus the water kicked up by the water taxi has the signature of the salt in the water.
- "Strangers on a Train"-Plot Murder: In this case, Strangers on a Boat. Castle even mentions the original novel by Patricia Higsmith and makes the "criss-cross" gesture.
- Tempting Fate: Both Ryan and Esposito and Castle have a lot of moments where they have an obvious suspect and rub it in a little, only for that suspect to be cleared by some kind of alibi or forensic evidence.
