An ER doctors is chased and shot to death by someone dressed as Santa Claus. The team at the 12th precinct figure out that the doctor also worked for the mob. Castle turns out to know the mobster. It's Dino Scarpella whom Castle has gone to for help with research in the past. Dino's convinced that Beckett has arrested the wrong man and tasks Castle with making things right. Rick's investigation leads to uncovering that Dino's daughter is in love with Luca who is in a rival mob family. But Luca didn't kill Eric. The murder leads right back to someone inside the NYPD and has dire consequences for Rick Castle's future as Beckett's partner.

Elsewhere, Dr. Lanie Parish's parents visit town and it turns out that Lanie's been lying to them to get them off her back. She told mom and dad that she and Esposito were engaged. Now she needs Javi to play the role of her fiance. But when he steps up to the plate, it turns out to be far more complicated than he expected.

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