Castle (2009)

S4E14 The Blue Butterfly

As Castle and Beckett delve into the murder of a treasure hunter, they uncover a connection to a puzzling homicide from 1947 that centers around a tough private investigator. Castle understands that to crack the current case, they must first unravel the mystery of the earlier crime. The 1947 investigation unfolds through artistic flashbacks, showcasing Castle in the role of the private eye and Beckett as a seductive femme fatale.