When a beautiful woman is found dead in an alley, all clues point to her being a victim of the notorious Triple Killer. The Triple Killer, or 3XK, terrorized New York City four years ago, then mysteriously disappeared. Now he’s back. In a deadly battle of wits, Castle and Beckett race against time to stop 3XK before he claims his next victims.

Marshall and Lily try to figure out how to conceive a baby with the gender of their choice; Robin’s perky new co-anchor irritates her.

Ria blames herself when two museum security guards, that she vetted and approved, try to steal a valuable artifact. Both men are shot and killed by another guard, who is devastated that he killed the two men. Meanwhile, Cal becomes involved in a game of cat-and-mouse with a beautiful woman and the man who is stalking her.

Patrick Jane and the CBI team hit the horse track when a jockey is found murdered and suspects abound.

In an attempt to reconnect the family, Phil and Claire challenge them to see who can abstain from using their electronic devices the longest. Mitchell and Cameron attempt to get Lilly into the best pre-school. Gloria deals with the neighbor’s barking dog.

Against the family’s wishes, Frankie invites a Japanese foreign exchange student to live with them, in the hopes of exposing him to American life; however, the Hecks get frustrated when he shows no interest in them or America.

Alicia risks disbarment when Glenn’s office suspects her of an ethics violation, which could also ruin Peter’s political campaign. A third candidate is revealed in the campaign for state’s attorney.