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

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 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 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.

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 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 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

When a bomb explodes at a protest rally killing five people, Castle and Beckett realize the key to solving the case is to reconstruct the 47 seconds prior to the explosion. Using video and witness accounts, they uncover the shocking truth about who planted the bomb.

Castle and Beckett uncover that a bullet is missing from the crime scene while investigating what happened when an art dealer was gunned down in his own gallery. However, when it turns out there was a witness to the murder, they need to refresh the man's memory, only to find out that he suffers from amnesia and can't even remember who he is.

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

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.

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.

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.

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 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.

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.

Beckett is recruited to assist on a Narcotics investigation, but a routine mission turns deadly when an undercover operation goes array. To survive, Beckett must engage in a battle of wits against some of the most dangerous men in the New York underworld – including a powerful enemy from her past.

Mysteries abound when a family returns from vacation to discover a dead man in their daughter's bed. Thinking they're investigating the murder of a squatter, Castle and Beckett soon find themselves unraveling a multi-million dollar heist. Meanwhile, Castle tries to keep Beckett from finding out that they've been romantically linked when he's featured as one of New York's 10 most eligible bachelors.

Castle and Beckett investigate the murder of a dockworker turned bar owner who's body is found floating in the East River. In the process of the investigation they uncover a secret buried since the days of Prohibition.

Castle and Beckett pursue a ruthless killer who, they discover, is part of a sinister, international conspiracy. Castle and Beckett are joined on the case by Sophia Conrad, a smart, sexy CIA operative who has a complicated past with Castle.

With tensions rising between Castle and Beckett, Castle finds himself a new partner -- Detective Ethan Slaughter (guest star Adam Baldwin, "Firefly"), a tough and colorful cop from the city's gang unit. But Castle gets more than he bargained for when Slaughter shows him a grittier side of police work that puts Castle in the line of fire like never before

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.

After being banned from the 12th precinct, Castle works his first case as a private investigator.

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 look into the murder of a young mathematician gunned down with a 200-year-old bullet, sparking some wild theories by Castle about a time-traveling killer. When their investigation uncovers a tricked-out DeLorean and a suspect in Victorian clothing, is it possible Castle's crazy idea has some validity? This winding tale twists through the towers of Wall Street and into the world of a secret steampunk society that embraces the romance and simplicity of the past, while coupling it with the hope and promise of the future.