A treasure hunter named Stan is murdered while searching for the Blue Butterfly necklace.
Stan bought and searched for clues to lead him to the necklace of a woman who was the girlfriend of a 1947 mob boss. A PI had been hired to search for those woman, who he quickly fell for.
The PI and girlfriend plan to run away together but are approached by a former girlfriend’s daughter who wants her dead. A scuffle keeps the two from dying, and leads them to live a life together under different names.
Castle imagines himself as the investigator and Beckett as his girlfriend. Espo, Ryan, and Lanie play parts involved in the romance.
Clues lead them to a nurse caring for the couple who was also on the hunt for the necklace. He admits to killing Stan to get the treasure first.