A treasure hunter is murdered while investigating a long-abandoned nightclub in New York City. He was one of the many people searching for the Blue Butterfly, a valuable necklace which mysteriously disappeared in 1947. The treasure hunter had recently acquired the diary of Joe Flynn, a private detective who was supposedly murdered outside the nightclub in 1947. Castle uses the diary to retrace the last days in the life of Flynn, to attempt to solve a double homicide from 1947, and to locate the missing necklace. Meanwhile, he and Beckett investigate the current murder case, and interview the various people who had recent contact with the victim.
—Dimos I