When the murder of an Army veteran puts Beckett on the trail of the man who shot her, Castle must decide how much he's willing to sacrifice to keep her safe. As secrets are revealed and feelings are put on the table, the lives of the detectives at the 12th Precinct may never be the same.

In the powerful conclusion of a two-episode story arc, when the FBI fails to get his daughter back, Castle takes matters into his own hands, reaching out to a shadowy fixer to help him recover Alexis. But Castle soon learns that his daughter's kidnapping may be part of an even more sinister agenda.

Kate Beckett's investigation into her mother's death is reactivated when Hal Lockwood - a professional hitman with knowledge of Beckett's mother's murder - stages a spectacular escape during a court hearing. As police launch a manhunt for Lockwood, Castle and the team uncover evidence of a deeper conspiracy involving a mysterious figure with law enforcement connections who is linked to both Lockwood's escape and the killing of Johanna Beckett. The search for Lockwood and the quest to identify his co-conspirator leads to a shocking and deadly conclusion.

While investigating a shocking ritualistic murder, Beckett and her team uncover surprising evidence linking Castle to the killing. As the evidence against him mounts, loyalties are tested. And when surprising revelations come to light, Beckett begins to wonder how well she really knows her new lover and partner of four years

In the first of a two-episode story arc, Castle and Beckett begin a murder investigation that also exposes a plot to kidnap the daughter of a wealthy Middle Eastern businessman. FBI Agent Harris, an even-keeled professional is brought in to help find the kidnappers.

In the show’s 100th episode, Castle finds himself in the unfortunate position of NOT being able to work a case. While he’s holed up in the loft with a broken leg, Beckett goes off to investigate the death of an IRS agent without him, leaving Castle miserable and bored. But when he thinks he witnesses the murder of a young woman in the apartment across the street, he is thrown into a "Rear Window"-esque murder mystery that hits much too close to home.

Stakes rise as the 12th Precinct matches wits with serial killers Jerry Tyson (3XK) and Dr. Kelly Nieman. With the clock ticking, Castle and the team must crack the case before the killers claim their next victim.

Federal Agent Jordan Shaw, an insightful and accomplished Federal investigator teams up with Castle & Beckett to hunt down a cunning and elusive serial killer.

When the detective who investigated Beckett's mother's murder is shot before Beckett's eyes before he can give her new evidence in the case, Beckett's old wounds are reopened. The investigation of the detective's death leads the team to an old scheme which may provide the answer to why Beckett's mother was murdered and who ordered it. Beckett and Castle's relationship takes a new turn.

A crew of bank robbers disguised as classic television doctors take hostages which include Castle and Martha. But things are complicated by the fact that Beckett's on the outside having her every move scrutinized by a hostage negotiator and Castle's on the inside with information that the hostages are just a distraction for a more ominous plan.

Picking up the morning after, Castle and Beckett now have to decide if they're a couple or was it just a onetime deal. But before they can tackle that, they must deal with those responsible for Kate's mother's death who are now a threat to Beckett.

A secret investigation conducted by Beckett related to her mother's murder goes awry when a man she has been surveilling is killed. When evidence links Beckett to the victim, she and Castle become the targets of a manhunt that puts both of them in grave danger.

In the second part of the story begun in "Tick, Tick, Tick," the serial killer remains at large and is taunting Castle, Beckett and Jordan Shaw to catch him

Castle and Beckett arrive at a crime scene to discover that the female victim bears a shocking resemblance to Medical Examiner Lanie Parish. As the team tries to make sense of this mystery, they realize the killer may be targeting detectives of the 12th Precinct. Dr. Kelly Nieman, an exclusive Manhattan cosmetic surgeon who draws Castle and Beckett's interest because of her connection to the victim.

When clues in a murder implicate Castle and Beckett's nemesis, Dr. Kelly Nieman, Castle is called in to consult on the case.

With time running out, Castle and Beckett must put aside their differences with Agent Fallon to avert a city-wide catastrophe.

When Castle and Beckett investigate the killing of a treasure hunter, they discover the case is linked to a mysterious homicide from 1947 involving a hard-boiled private detective. Castle realizes the only way to solve the present-day murder is to solve the murder from the past. The 1947 case comes to life through stylized flashbacks, featuring Castle as the private eye and Beckett as a femme fatale.

Castle and Beckett’s plans for a romantic weekend in the Hamptons are interrupted when a dying man stumbles into Castle’s backyard, collapsing into his pool. Unable to escape the lure of the investigation they join up with the small town’s well-intentioned, but inexperienced sheriff to find the killer,

A grisly murder investigation leads Castle and Beckett to a suspect who claims he's traveled back in time to stop terrible events from unfolding, events that will change the course of human history. Is he simply a deluded killer, or is it possible that he's telling the truth?

While investigating the death of an Irish mobster, Beckett and Castle uncover a local turf war that may be connected to both a late-night infomercial guru and an international drug smuggling operation. But the case turns agonizingly personal when evidence emerges that the killer they're chasing may have had a hand in Beckett's mother's murder.

When a beautiful woman is found dead in an alley, all clues point to her being a victim of the notorious Triple Killer. The Triple Killer, or 3XK, terrorized New York City four years ago, then mysteriously disappeared. Now he's back. In a deadly battle of wits, Castle and Beckett race against time to stop 3XK before he claims his next victims.

When Castle's novel ''Heat Wave'' becomes a movie, actress Natalie Rhodes accepts Beckett's invitation to visit and learn about the character she will play; Natalie observes Castle and Beckett's investigation of the murder of a matchmaker.

When a body is found at a sci-fi convention, Castle and Beckett are warped into the heart of fandom -- a world of egotistic celebrities, diehard costumed fans and enough drama for two galaxies.

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 takes over the investigation. Together, Fallon, Beckett and Castle must race to unmask the truth about the victim before it's too late.

When Castle and Beckett wake up in bed, handcuffed together, in a locked room with no memory of how they got there, they must piece together the mystery of where they are and why, all while trying to escape. But as the two of them adjust to being shackled together, what's the biggest threat they face... the people who abducted them or each other? Meanwhile, Ryan, Esposito and Gates are left to investigate their disappearance in a case where all is not what it seems.