New York City, 1947. Songbird Betsy Sinclair belts out a ballad as P.I. Joe Flynn downs a drink at the Pennybaker Club. The fedora-wearing detective is looking for a dame named Vera Mulqueen. She’s a striking beauty who just happens to look a lot like Beckett. Betsy Sinclair, by the way, is a dead ringer for Lanie. As for Joe Flynn, he bears a striking resemblance to Castle. As you may have guessed, this flashback to the ‘40s has something to do with a present-day murder.

Stan Banks was shot to death in the now-abandoned Pennybaker Club. Stan’s wife says they’ve been separated for about a year after her hubby became a treasure hunter. He was obsessed with finding something called the Blue Butterfly. Castle finds a 1940s diary amongst Stan’s stuff. It belonged to a private eye named Joe Flynn. Castle can’t resist reading the juicy journal. It could be the key to the case. Plus, it’s really cool.

On June 18, 1947, secretary Florence Kennard (looking a lot like Martha) tells Joe Flynn they have a potential new client. Sally Scofield (that would be Alexis) steps into the office in her t-strapped shoes and country suit. She talked her new husband into honeymooning in the Big Apple so she could track down her sister, Vera Mulqueen. She wants Joe Flynn to find this beautiful doll.

When Joe tracks down Vera at the Pennybaker Club, he learns that she’s ruthless mob man Tom Dempsey’s main dame. Two henchmen (picture Ryan and Esposito) teach Joe a lesson about eyeballing someone else’s lady. Vera quickly learns that the handsome P.I. can take a punch. As for Joe, he can see that Vera looks even more stunning thanks to a blue diamond-studded necklace. In the present, Castle exclaims, “The Blue Butterfly! It’s a necklace! That’s why Stan Banks was killed.” He then asks, “Why am I narrating?” Classic stuff.