A beautiful woman is found hanging in a park by leather cuffs while covered in caramel. Beckett notices that the cuffs are custom-made and may be easy to trace. Castle wants to know how she knows the bondage bracelets are custom-made. Frankly, we're wondering the same thing. Sure enough, Barry from The Love Shackle (great name for a store) identifies the cuffs as his design. They were purchased by Tyler Benton and were made for his girlfriend, Jessica Margolis.
Tyler is devastated when he learns about Jessica's death. He bought the cuffs as a joke because Jessica was doing research on the sociology of sexual bondage and domination for her PhD. Jessica's roommate, Danielle, hadn't seen the cuffs since Tyler gave them to her roomie.
Castle and Beckett meet Kelly, Jessica's officemate, and Matt Haley, another PhD student at Hudson University. Matt admits to a rivalry with Jessica, but insists they were civil to each other. According to Jessica's thesis proposal, she became interested in S&M after witnessing a dysfunctional relationship in her own life.
Jessica's journals indicate that she was shadowing a dominatrix named Mistress Venom during her sessions at Lady Irena's House of Pain (another great name). Beckett sets up an appointment with Mistress Venom with her “boyfriend Ricky” who has been very bad. But Lady Irena says Jessica wasn't observing Mistress Venom, she WAS Mistress Venom.
Another dominatrix, Mistress Sapphire, says she saw Jessica crying after a first-time client came in and left suddenly. He was wearing medical scrubs and Jessica's boyfriend, Tyler, works at a hospital. But Tyler's not the killer. Neither is William Caraway, one of Mistress Venom's clients who thought she should be drowned in caramel. Caraway has an alibi which means someone with access to Jessica's notes was trying to frame him.
At Hudson University, Castle finds a picture of Matt Haley wearing scrubs as part of his PhD research. Matt admits that he went to the dungeon to blackmail Jessica into dropping out of the running for a fellowship, which she did. There goes his motive. At Jessica's apartment, Danielle shows Beckett and Castle a wine glass with lipstick marks. Castle recognizes it as Lady Irena's shade.
Jessica was supposedly seeing other clients outside of Lady Irena's dungeon, cheating her boss out of her cut. Looks like Lady Irena came over to confront Jessica and things got out of hand. The floor in Jessica's room was scrubbed with bleach and dried saliva was found in one of Jessica's pillows, indicating she was smothered.
Another case seems to be in the books, until Castle has a revelation: If CSU found saliva on the pillow but not the pillowcase, who cleaned the pillowcase? Irena wouldn't have had enough time to do all that. Only one person could have cleaned the apartment: Danielle.
Beckett theorizes that the first-hand dysfunctional relationship Jessica wrote about was between her and Danielle, who read everything Jessica wrote about her. Faced with the truth, Danielle finally admits that when Jessica said she wanted to move out, she got angry and pushed her. Jessica hit her head and screamed that she hated Danielle. All Danielle wanted to do was stop the screaming. And she did. Case closed.
At the end of the shift, Ryan's girlfriend, Jenny, comes by the precinct to meet the team for the first time. She's a beautiful and charming woman. Beckett thinks Ryan kept her hidden for so long because he was afraid they'd embarrass him. Esposito says it's actually so he and Castle wouldn't try to steal her from him.
As Castle and Beckett walk off, they muse about the psychology of domination and submission and the inevitable comparison to their own relationship. Castle thinks Beckett's the punisher and he's punishee. But Beckett has a list of grievances that may prove otherwise. It started the first day they met...