Castle reconnects with a former girlfriend (Alyssa Milano) on her wedding day when one of her bridesmaids is found murdered, causing sparks to fly between them once again, and Beckett to have to deal with the possibility of being jealous.
- Amicable Exes:
- Castle and Kyra seem to have long since put aside any lingering resentment and are generally wistful about their former relationship, although both ultimately reject any idea of rekindling it.
- Greg and Sophie used to date before he met Kyra. Sophie seemed to be over him, even having introduced Kyra and Greg, but still carried a torch, which Greg's uncle exploited to try and break up the wedding.
- Bait-and-Switch Accusation: At first, Castle thinks Beckett's pissed at him for seeing Kyra when she told him not to, only for her to reveal she's pissed he messed with her chair. Then she reveals she had him followed and knows he saw Kyra the night before, and is kinda pissed about that, too.
- Evil Uncle: Ted initially seems like a Cool Uncle who is supportive of Greg and is implied to be the Life of the Party, but turns out to have been embezzling from his nephew's trust fund for years to pay for frivolous luxuries, and shows no hesitations about trying to ruin Greg's wedding to avoid exposure.
- Explain, Explain... Oh, Crap!: Greg defends Ted from the murder accusation against him by citing all of the good things he's done for him, but in the process, realizes why his uncle did it.
- Foreshadowing: Ryan is able to translate the technical name for roofies, showcasing his time as a narc.
- A Man Is Always Eager: Discussed after Greg's best man doesn't show up for interviews and is found in a closet, having been drugged and left there by Sophie. This puzzles everyone; Castle and the best man both point out that roofieing a man is counterproductive to sleeping with him, and "all a girl needs to do is ask". In fact, what Sophie really wanted was access to his room, which adjoined Greg's.
- Meaningful Name: Castle's serious ex, Kyra Blaine, shares initials with Kate Beckett.
- Mistaken Confession: Double Subverted. When Beckett chews out Castle for doing something she told him not to and yanking on things he wasn't supposed to, Castle thinks Beckett found out he saw Kyra (a suspect in the case) after she warned him against it and tries to stammer out explanations when actually Beckett elaborates that she's talking about using her chair (and messing with the height nob) before Castle blurts out anything really incriminating. After a short pause, he confesses to seeing Kyra though, and Beckett admits that she'd already known.
- The Missus and the Ex: A twofer. Castle is the Ex to Greg's Missus through Kyra. Kyra is the ex to Kate's Missus through Castle.
- O.O.C. Is Serious Business: Kate lampshades that Castle isn't coming up with theories about one of the most obvious suspects because she's his ex.
- Obnoxious In-Laws: Discussed. Kyra and Castle weren't actually married, but her mother had this kind of relationship with Castle when they were dating. Upon seeing her again, he quickly and smugly points out that he is not homeless or a teacher at a third-rate college like she'd predicted, while she continues throwing barbs at him. Beckett tells him to imagine how he'd have had to spend Thanksgivings with her if he and Kyra had made it work, causing Castle to shudder.
- The One That Got Away: Kyra to Castle, who describes her as this word-for-word.
- Seduction-Proof Marriage: Ted hired Sophie to try to seduce Greg and get the wedding called off, but he refused to cheat on Kyra and threw Sophie off of his bed, causing her to flee. Castle plays Kyra the tape Sophie made, in which Greg clearly states he loves Kyra and would never cheat on her, and tells her that Greg's fidelity is the only reason his uncle's plan didn't work.
- She Knows Too Much: After Sophie failed to seduce Greg, she had a change of heart and threatened to expose how Ted has been stealing from his nephew's trust fund. So he resorted to another way to stop the wedding: kill a bridesmaid.
- Suspicious Spending: Kyra's mother mentions that Sophie had tried to borrow money for her plane ticket out there, but then had gone and bought her own bridesmaid dress. Further checking shows she got other expensive items for the wedding, and had paid three months' rent despite having only $118 ($100 of which she spent on the flight) in her bank account. The check further reveals that she got that money from Ted.
