Here's a bit about the episode:
CASTLE - "The Blue Butterfly" - When Castle and Beckett investigate the killing of a treasure hunter, they discover the case is linked to a mysterious homicide from 1947 involving a hard-boiled private detective. Castle realizes the only way to solve the present-day murder is to solve the murder from the past. The 1947 case comes to life through stylized flashbacks, featuring Castle as the private eye and Beckett as a femme fatale, on "Castle," MONDAY, FEBRUARY 6 (10:01-11:00 p.m., ET), on the ABC Television Network. (ABC/KAREN NEAL) STANA KATIC, NATHAN FILLION
And here are a few of the promo shots ABC released -- aren't these absolutely AMAZING?! The rest of the set can be viewed here.