Castle and Beckett learn that The Fist of Capitalism is a sculpture worth $50 million-30 of that is in diamonds. Bryan Hayes, the executive director of the museum where it was being housed, is dead and The Fist of Capitalism is gone. Catching this killer will be like finding lots of stolen diamonds in the rough.
The killer struck during a museum gala. Myrna Ramsey, the development director, says the security system had been disabled. Museum worker/budding artist Alyssa Lofters says Bryan Hayes was a lovely man who allowed her to put one of her sculptures in the exhibit. He also feared someone might steal the Fist of Capitalism.
As Castle and Beckett theorize about the crime, Serena Kaye (guest star Kristin Lehman) gives them the 411as to how the thief made his way into the museum. Who is Serena Kaye, you ask? She happens to be a sexy insurance investigator who will be acting as a consultant on the case. Beckett doesn't like it. Of course, that may be because Serena isn't hiding her attraction to Castle.
Serena believes the M.O. of the crime fits the one used by Falco, a top art thief in Europe. He's never been caught, never been photographed. Castle tags along as Serena meets with a secret informant who suggests Falco was in cahoots with the victim, Bryan Hayes. But the sculpture's co-owner, Joy McHugh, thinks her soon-to-be ex-hubby, Anton, is the thief.
Castle is smitten with the sexy Serena Kaye. Beckett... not so much. She's struggling with the fact that her partner is interested in another woman. Beckett's therapist asks, "What are you really scared of? That he won't wait for you? Or that he will?" Excellent question. As for the case, the team learns that Bryan Hayes recently had a lunch meeting with Serena. Beckett believes she the security investigator is actually their thief.
Castle keeps Serena occupied during a date while Beckett and Esposito search her hotel room. During dinner, Serena admits that she used to be an art thief. Actually, she "retrieves" pieces only to return them to their rightful owners. Castle believes she's like a female Indiana Jones, but without the hat and whip. Serena corrects, "Well, without the hat."
Castle tries to warn Beckett that he and Serena are headed their way. The only way distract Serena is to plant a kiss on her in the hallway. As it turns out, the sudden smooch wasn't necessary. Beckett and Esposito found robbery evidence in the hotel room. Serena is under arrest and Castle is in the doghouse. Beckett is upset because he doesn't believe Serena is guilty. She's also not all that thrilled about that impromptu kiss.
Beckett eventually realizes Serena isn't a killer. She met with Bryan Hayes over security concerns and wasn't aware that the guy may have been in on the crime. In fact, Serena is able to ID Falco on a security tape. Traces of hair gel were found in a museum access vent as well as the getaway van used during the theft. The team tracks the gel to one made for an exclusive hotel. A short time later, the elusive Falco is arrested in his room.
Falco is a crook, not a killer. Serena has a thief-to-thief chat with the guy. He was only hired to test security by disabling the alarm. Still, the allure of the valuable art piece was temping. But Falco was scared away when he saw a woman in a black dress walking toward the sculpture.
Alyssa Lofters was dressed in black that night. Castle kicks open the base of Alyssa's sculpture to find the Fist of Capitalism hidden inside. But the artist no longer owned the sculpture. She had sold it a week ago to the real thief and killer, Joy McHugh, who was trying to keep it from her hubby. Case closed.
Beckett tells Serena that Castle believed she was innocent all along. She wanted her to know what kind of guy he is. It's a nice gesture, but an unnecessary one. Serena is no longer interested in Castle. She says, "I don't steal things that belong to someone else." Beckett is surprised to see that Serena and Castle didn't go out. As consolation, she offers to buy her partner a burger. Castle accepts her offer, as we all knew he would.