The episode opens with a body falling out of a hotel room window. Meanwhile, Ryan and Jenny are knee-deep in wedding plans. Everyone is responding to their invites and Ryan tells Esposito that Lanie is bringing someone as a plus-one.
The team are at the crime scene of Michael Bailey's death, and evidence suggests that he had sex before he fell out of the window. Therefore Castle suggests that this could be murder. Maybe a woman is having sex with her victims before killing them. The only other fingerprints found in the hotel room belong to a Ms. Franklin, who is shocked to discover that Bailey is dead. However, she didn't know the victim as Michael, but Jake. Also, she claims that she did witness Michael fall, but did not kill him. He was already dead before the fall. She thinks that Bailey's protein shake was poisoned.
When the team tries to track down everyone Bailey had a relationship with, they find out that he had a great deal of partners. However, he did tell them all his real name, all except Ms. Franklin. He also told his partners that he would be leaving town for a while, but he apparently stayed. He did abandon his job at a company as a systems analyst, left his apartment, and had a lot of money with him.
Esposito discovers that Lanie's date for the wedding, Toby Lang, has an impressive resume, as well as impressive looks. Espositois now desperate to have a plus-one of his own, but who should he ask? Meanwhile, the team tracks down one of Bailey's other girlfriends, Colette Roth. Colette tells them that she watched Bailey get kidnapped, but the team doesn't believe it until security cam footage of the incident turns up.
Castle suspects that Bailey was some sort of secret agent, but no one at the CIA or FBI will admit that he's working for them. Ryan tracks down the owners of the car that Michael was dragged into and the team finds out that Michael Bailey worked with two other men as pick-up artists. The kidnapping was a ruse to scare away Colette, because she wanted to get too close to Michael.
At the time of Michael's death, he was trying to woo a woman named Lisa, who is a publishing heiress, and married. Meanwhile Castle finds out that Michael slept with Jenny after she and Ryan had started dating. Castle wonders whether he should tell Ryan the truth.
It looks like both Colette and Lisa were rich women, and that Michael and the two other guys were running an extortion scheme, demanding money so that footage of their affairs wouldn't get leaked. Lisa is married to Ron Hill, who is an investment banker trying to broker a deal with Michael's company. Michael was trying to look up some insider info on Ron's computer while having the affair with Lisa.
Ryan casually tells the team that he already knew about Jenny sleeping with Michael. And he is completely okay with it. He goes with Esposito to check out the place Michael had been staying in, and notice that there are a lot of pre-made protein shakes. The shakes were courtesy of a woman named Amy Camp, who is COO of Michael's company. She was also seduced by Michael and he got a lot of money from her. But she claims that it was less about vengeance and more about business.
The episode closes at Ryan and Jenny's wedding. Lanie's date is handsome, but gay. Esposito has brought along a date as well, his beautiful cousin. Castle and Beckett discuss the wedding and they're pleased to see Ryan so happy. Actually Castle admits to some jealousy. Beckett wonders whether the third time may be the charm for Castle.