When Castle and Beckett are called to a wedding to investigate the death of a bridesmaid, Castle is stunned to discover that the soon-to-be bride is his old flame, Kyra, “the one that got away” (guest starring Alyssa Milano). Complications ensue as Kyra, caught off-guard by Castle’s reappearance in her life, wonders if she’s marrying the right guy. With the nuptials forestalled and the entire wedding party under suspicion, Beckett questions Castle’s ability to be objective about the case. But when it looks to Beckett like Castle and Kyra might rekindle their romance, her judgment may be compromised as well.
Nothing is as it seems when Castle and Beckett investigate what appears to be the assassination of an intelligence operative. Meanwhile, Beckett's romance with Demming threatens her relationship with Castle, forcing them to confront their feelings for each other.
Castle and Beckett investigate the hit-and-run death of a bike messenger, brutally murdered to prevent him from delivering a package, the contents of which could free a wrongly convicted man from prison. In a surprising twist, this tragic death is linked to a murder case that Captain Montgomery investigated a decade earlier. To solve the present murder, Castle and Beckett must dredge up the past by uncovering the secrets and scandals of a prominent New York family.
A high profile chef is found frozen to death in the kitchen of a hot New York restaurant. Complications ensue when Castle and the restaurant owner, an old high school girlfriend of Beckett's, share an attraction. Meanwhile, Beckett's relationship with Det. Demming continues to grow.
When an Arctic explorer is killed during a polar expedition, the mysterious circumstances of his death take Castle and Beckett from the penthouses of Park Avenue to the halls of the CIA. But Castle’s delight at the case’s many twists and turns soon turns to frustration as he realizes that their elusive killer might just be fooling them all.
Castle’s book agent, Paula Haas (guest star Debi Mazar), approaches him with a career changing opportunity, but taking it would mean the end of his relationship with Beckett. Will the murder of an unidentified woman found dead in a manhole be their last case together?
The murder of an Assistant District Attorney thrusts Castle and Beckett into the world of ex-cons and escorts. But when Castle’s desire to help a vulnerable call girl endangers the case, Beckett is forced to decide between honoring promises and putting a killer behind bars. Matters are complicated even further when Castle’s daughter shares a potentially explosive secret with Beckett instead of her father.
When an art dealer is gunned down in his gallery, Castle & Beckett discover that a bullet is inexplicably missing from the crime scene. The mystery is deepened by an amnesiac who witnessed the crime but can’t remember what he saw or even who he is.
When an up-and-coming rock star’s dead body is found staged in a scene straight out of her hit music video, Castle and Beckett delve into the world of obsessed fans, sleazy managers and jealous band members. But it’s only when Alexis uncovers the true meaning behind the lyrics of the final song that clues to the killer’s identity begin to surface.
When a murdered man is found stuffed in the garbage chute of an apartment building and two women arrive to ID his body — one his wife and the other his fiance — Castle and Beckett have their hands full keeping the women from killing each other as they work to peel back the layers of the victim’s double life. Was it a jilted woman, corporate espionage, or a much darker secret that led to his demise?