Following four seasons of sexual tension between Castle (Nathan Fillion) and Beckett (Stana Katic), the fifth season opens with the two of them finally embarking on a romantic relationship together. However, they try to keep this new relationship a secret, particularly from their boss, Capt. Victoria Gates (Penny Johnson Jerald). Also, Beckett finally discovers who's responsible for her mother's murder and wrestles with whether she should carry out her personal vendetta against him. Likewise, an emotional Castle finds himself pursuing criminals when his daughter, Alexis (Molly Quinn), is kidnapped, and he receives help from an unlikely source: his estranged father (James Brolin). Several episodes this season give nods to pop culture, including Fillion's role on "Firefly," which is alluded to when Castle and Beckett investigate a murder at a sci-fi convention; and the show's 100th episode, which pays homage to "Rear Window" when an injured and housebound Castle believes he's witnessed a murder.