The Blue Butterfly

Plot:

Castle senses a diary of a P.I from the 1940s found in the effects of a murdered modern-day treasure hunter is the key to finding the Blue Butterfly, a blue-diamond necklace. The team works out the story situated in 1947 in the same, now derelict, music club, involving murder, gangsters, and theft; they work out the true parts and identities of culprits and victims.