Castle's novel "Heat Wave" is being turned into a movie, and the Hollywood hottie cast as the movie's lead shows up at Beckett's invitation to better understand the character she's going to play. Laura Prepon guest stars as actress Natalie Rhodes. As our threesome investigates the murder of a high-class matchmaker, Natalie goes to extremes in the name of "character research," transforming herself into the spitting image of Beckett -- much to Castle's delight and Beckett's dismay.