Castle gets trapped in an alternate universe where Beckett is a captain and has never met Castle. And the two finally get married.
- Alternate Universe/It's a Wonderful Plot: An Incan artifact sends Castle into an alternate universe. In this universe, he's never met Beckett. Because he never met Beckett, several things are different. Castle never wrote the Nikki Heat novels and instead wrote uninspired dreck. Beckett never solved her mother's murder, so she eventually became captain and never softened. Because Castle stopped making money, Martha had to actually work and goes back to Broadway. Alexis moves to LA to live with her mother because Castle has given up. Lanie is pregnant with Esposito's child. Ryan never got married to Jenny due to increased caseload. And Alexis is a brunette! Oh, and the break room coffee is as awful as it was in the pilot.
- Maybe Magic, Maybe Mundane: Perhaps it really was the Incan artifact, or perhaps It Was All A Dream...
- At the end of the episode, Beckett arrests the same three killers that Castle encountered while in the alternate universe. Since he had never seen them in the standard Castle universe, that strongly suggests that the trip to the Alternate Universe was real.
- They Do