In the Castle episode " The Limey " (S4E20), a Scotland Yard detective helps Castle and Beckett solve a British model's murder, which leads to a major shift in the main characters' relationship. As Beckett finally admits her feelings for Castle to Lanie, Castle starts dating someone else, leaving them at a crossroads. You can read a full recap of the episode on TV Fanatic or check the episode details on IMDb . Castle: Episode 4x20 The Limey - Script Line