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.

This release compiles all 23 episodes from the fourth season of Castle. This run of shows finds Beckett getting closer to the truth about her mother's death, as Castle grows more protective of her.

The sixth season of this crime drama opens with Beckett (Stana Katic) accepting a marriage proposal from Castle (Nathan Fillion). However, their road to the altar gets a little bumpy when Beckett also accepts a new high-profile job in Washington, D.C. Meanwhile, things don't go smoothly for Castle and Alexis (Molly Quinn) when he doesn't approve of the new boyfriend she's brought home from Costa Rica. Castle gets a glimpse at what parenthood would be like a second time when he and Beckett temporarily care for a baby who was carried into a church by a man dying from gunshot wounds. And Ryan (Seamus Dever) and his wife excitedly prepare for their first child, but she goes into labor while Ryan is investigating a dangerous arson case.