When a coded document hidden by a murdered bail bondsman gets Castle's imagination running wild, Beckett must steer the case back to the violent felons who populated their victim's world - only to discover that maybe Castle's crazy theories aren't so crazy after all. Complicating matters is the surprise arrival of Beckett's ex-partner (now a bounty hunter). As the two reunite over a history that Castle has no part of, Castle is forced to watch the relationship take a turn towards the romantic.
Castle and Beckett investigate the apparent robbery-homicide of a New York cab driver, only to discover that the victim was involved in a mysterious conspiracy with unimaginable consequences. As the stakes continue to rise, hard-charging federal agent Mark Fallon (guest star Adrian Pasdar) takes over the investigation. Together, Fallon, Beckett and Castle must race to unmask the truth about the victim before it's too late.
While investigating the shooting death of her ex-partner in New York, Beckett comes to believe the suspect has fled to Los Angeles. Against orders, she travels to L.A., going renegade to track down the killer, with Castle unexpectedly along for the ride. As they dig deeper into the motives, they uncover a trail that leads from a rooftop pool in Hollywood to the sandy beach of Santa Monica. Along the way, they visit the set of the "Heat Wave" movie (based on Castle's book) and enlist the cast to help break the case. But as they get closer to apprehending the killer, Castle begins to wonder whether Beckett is out for justice -- or revenge.
In "Limelight," Castle and Beckett investigate the murder of a young woman.
When an ex-con is mysteriously slain in an alley, Castle and Beckett believe a vigilante is behind the murder. But efforts to identify their suspect are thwarted when they discover that he roams the city in a Superhero costume — and may indeed be a Superhero. Can they capture and unmask the killer before he strikes again?
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?
Castle thinks the Antichrist maybe involved in the death of an axe wielding psychiatric inmate who drops dead in the P.I. office.
With Beckett still in D.C., Castle and the boys investigate the murder of former child star Charlie Reynolds, who rose to fame playing the lovable nerd on an early 90s High School sitcom.
Castle is hired to uncover if a client’s husband is having an affair, but the case takes a shocking turn when Castle witnesses the client’s murder – by her husband. But with no proof that the killing took place, Castle and Beckett must investigate to expose the truth.
Detective Slaughter (guest star Adam Baldwin) returns to enlist Castle’s help in solving a high-stakes robbery case. But when a body is found linked to the crime, Slaughter becomes the number one suspect in the heist turned murder, while Castle has to determine whether he’s guilty or help to prove he’s innocent.