Get ready for a whole new Castle look! When Castle and Beckett investigate the murder of a treasure hunter, they discover that the case is linked to a 1947 homicide. Castle, of course, wants to solve the present murder by solving that of the past. So we get a look at a hard-boiled detective (that would be Castle), a femme fatale (that would be Beckett), a shady businessman (guest star Mark Pellegrino), and a whole lot of film noir. "The Blue Butterfly" will air on Monday, February 6 at 10pm on ABC.