Beckett, Castle and the Twelfth Precinct are called in to investigate a murder at an abandoned building that was once known as the Penny Baker Club back in The '40s. The victim, Stan Banks, is discovered to be an amateur treasure hunter looking for something he called, according to his ex-wife, "the blue butterfly". Upon knowing that Stan owes an unknown person ten thousand dollars, Beckett focuses on finding the only witness to the murder and pursuing the mystery man. But when those leads dry up, Castle happens upon more when he reads an old diary collected as evidence from Stan's apartment and comes across the story of its former owner, the private detective Joe Flynn who lived in the 1940s. Although Beckett is initially dismissive (as usual), Castle's gleeful anticipation of the tales of the past, especially of Joe's forbidden romance with gangster moll Vera Mulqueen, leads them directly to the murderer.
Joe and Vera's story involves the following tropes:
- Accidental Murder: Joe and Vera were able to disappear because Sally and her husband showed up with a gun to murder Vera. Sally shot her husband when Joe went to stop him taking the Butterfly, then Vera and Sally wrestled for the gun and Sally got a bullet in the belly. Joe and Vera put Sally and whatshisname in their car, torched it, and disappeared.
- Arc Words: The final scene in 1947 shows Beckett-as-Vera telling Castle-as-Joe "Always", the word that is always used for the modern day couple.
- Batman Gambit: Joe's plan to steal the Blue Butterfly and help Vera escape from Dempsey hinges on the fact Dempsey and his henchmen will be too focused on a radio broadcast of a wrestling match to notice she's left the club.
- Big Damn Kiss: Joe and Vera share one last kiss in the final scene before they go off to live their lives together.
- Bitch in Sheep's Clothing: Sally Scofield first appears as a tearful and sincere young woman who wants her older sister Vera home safe and sound and she employs Joe to find her. True to the narrative style of film noirs, her real motives and goals are far more disturbing and grislier.
- Breather Episode: Alongside "An Embarrassment of Bitches", this episode provides a gap between the arc-heavy episode of Beckett's mother before and the two-parter mid-season drama of "Pandora" and "Linchpin" that follow.
- But You Were There, and You, and You: Castle fantasizes the historical characters as written in Joe's diary as himself, Beckett, Martha, Alexis, Esposito, Ryan and Lanie.
- Chekhov's Gunman: The one who sold Joe's diary to Stan mentions having a son. Said son turns out be the culprit.
- Credits Gag: Instead of the usual music, the pen stabs its way down in the opening card with a classic jazz riff.
- Deliberate Values Dissonance: "You two need to keep this on the hush hush; Dempsey's not too keen on mixed laundry." After not-Lanie gives not-Castle an alibi by hopping off stage and smooching him.
- Earn Your Happy Ending: Joe and Vera got theirs when they started a family: children, grandchildren and great grandchildren. The elderly Joe muses why would he give up all that for a Blue Butterfly.
- Driven to Suicide: Vera's predecessor as Dempsey's moll, after Dempsey kicked her to the curb.
- Faking the Dead: Joe and Vera faked their deaths by burning the bodies of Sally and her husband in Joe's car beyond recognition, so the police assumed it was Joe and Vera in there.
- Foreshadowing: When the Irishman and the Cuban kick beat Joe up and kick him out of the Penny Baker the first time, he pulls himself upright on the wall of the back alley and a brick falls out. That loose brick ends up being the hiding place of the Blue Butterfly.
- Freudian Slip: "...and Kate's heart quickened".Beckett: Kate? You imagined yourself as the P.I. and me as the gangster's moll?
- Gorgeous Period Dress: The flashbacks take full advantage of the setting to put the cast, in Castle's imagination, in the most beautiful suits and gowns. Katic gets to wrap herself in furs, Fillion with hats and suspenders, Tamala Jones in shiny gowns.
- Homage: To the film noir classics of the 1940s.
- I Take Offence to That Last One: When Castle asks Ryan to intone the word "boyo" in the accent he imagines Dempsey's Irish henchman to speak in, Ryan obliges repeatedly until Castle says to make it sound like a leprechaun. Ryan snaps.
- Identical Grandson: Tom Dempsey in the 1940s and Tom Dempsey the Third in the present day.
- MacGuffin: The titular Blue Butterfly, in both the past and the present.
- Meaningful Name: Vera's last name is "Mulqueen". She's a moll and a queen.
- Not So Above It All: Everyone ends up getting drawn into the beautiful romance of Joe and Vera, thanks to Castle's love of the PI's diary and then the cold case box from the 40s.
- Oh, Crap!: Joe and Vera are terrified when Castle and Beckett show up and identify them by their real names.
- Revenge: Sally is in fact the daughter of Dempsey's previous moll who killed herself after being dumped. She hired Joe all so she can catch Vera to make her suffer like her mother did.
- Riddle for the Ages: What happened to the real Blue Butterfly? The one hidden away in 1947 was a fake, and the last reliable information concerning the whereabouts of the original necklace would place it somewhere in Nazi Germany before the end of World War II.
- Shipper on Deck: Castle becomes one for Joe and Vera when he starts reading their story from Joe's old diary. Beckett hops on the wagon herself later after hearing it from Castle, blowing up emotionally when there is no written conclusion resolving it.
- Shown Their Work: Joe and Vera in the present have the gnarly New York 40s accent and Southern Belle accent they had in the past, respectively.
- Sickeningly Sweethearts: Joe and Vera, in Betty Sinclair's opinion.Betty: You two are a walking fairytale. Good Lord.
- Verbal Backspace: After his Freudian Slip above, Castle tries to pretend he's not picturing the PI and the moll as himself and Beckett.Castle: What? No! I said "fate". "Fate's heart quickened." I was being poetical.
- Wham Line: When he's forced to reveal to Vera why he had been looking for her the night that they met, he tells her that her sister hired him to find her. An affronted yet confused Vera then informs him of something that makes Joe realise he's been had.Vera: I don't have a sister.
- You're Insane!: Joe's secretary spouts this about Joe and Vera's plan to steal The Blue Butterfly, along with a dose of What the Hell, Hero? to her own employer.
