Castle and Beckett are engaged. Beckett starts FBI training in Washington D.C. and is assigned to a case. Someone broke into a secured building and may have stolen codes. Castle flies to D.C. and is exposed to a deadly toxin. The code theft was a diversion.
A woman wakes to find her boyfriend murdered in her apartment. She eludes police and takes hostages and waits for Castle to help her. Castle believes she has been framed, and works with the team to prove her innocence.
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.
The team investigates the ritual murder of a woman. Symbols on the woman's body and the items left in her apartment seem reminiscent of The Da Vinci Code. Alexis and Pi have moved to their own apartment. Castle finds it hard to accept.
Alexis volunteers in a criminal review organization to help prove a man was wrongly accused of murder 15 years ago. With his execution scheduled in three days, Alexis asks her father to bring a new look at the evidence.
A man dies inside a church, protecting a baby. The team has to find the guy's killer and the baby's parents. The man is connected to a crime, but he protecting the baby does not fit a criminal character.
When a rising young pop star is found dead in an alley, Castle and Beckett delve into the victim's out-of-control hard-partying lifestyle, but as they dig deeper, a shocking revelation threatens to rock their investigation.
The murder of a PA at a top fashion magazine brings Castle and Beckett face to face with a tangled web of suspicion and corporate politics - and with the magazine's formidable editor, who still remembers Beckett's brief career as a model.
Castle and Beckett are baffled when a confused woman with an unshakable alibi gives a detailed and convincing confession to a murder. Then a second person walks into the precinct and makes the same confession. And then a third.
Beckett is recruited to assist on a Narcotics investigation, but a routine mission turns deadly when an undercover operation goes awry. To survive, Beckett must engage in a battle against some of the most dangerous men in the New York underworld - including a powerful enemy from her past.
Investigating the killing of a Japanese ballerina, Castle and Beckett find a shrine with the murder weapon in an old factory. Then, when the two are separated, Castle is ambushed by a hooded attacker bearing all the hallmarks of a ninja.
When construction workers uncover a mobster's body buried in concrete for 35 years, Castle and Beckett must try to talk to a fellow mafioso so stricken with grief that he lives as a recluse in a Seventies time-warp.
A man Beckett has been secretly following is found dead - and then a notorious criminal involved in the first death is killed with Beckett's own gun. Faced with an elaborate frame-up, she has no choice but to run.
The wait is finally over for Castle as the wedding day approaches. Kate discovers that she's still married to her college sweetheart, and visits him to get a divorce. But he has problems of his own, bigger than anyone could imagine.