Agents Beckett and McCord arrive to take over Castle and the guys' investigation into the death of a former child star, creating tension in the precinct.
Beckett races to find a stolen toxin capable of killing thousands of people; Castle has less than 24 hours to live after being exposed to the chemical weapon.
Beckett and Castle face the consequences of their decisions during a high-stakes investigation.
Season six begins where season five left off. Will Beckett accept Castle's proposal?
An interview with a federal law enforcement agency forces Beckett to reconsider her career and her relationship with Castle; a young woman's body is found inside a flop house's water tank.
When a government whistleblower is murdered, Beckett and Castle must figure out who is responsible and determine why Homeland Security is trying to take over the case.
When Beckett steps on a pressure-sensitive bomb, the team searches for a way to save her while Castle distracts her by talking about their relationship.
When Beckett is tasked with protecting a charming billionaire whose life is in danger, she reflects on her relationship with Castle; consumed with jealousy, Castle tries to solve the case as quickly as possible.
Beckett and Castle discover giant footprints at the site of a vicious murder, leading the team to wonder if Bigfoot is the killer.
While stuck at home with a broken leg, Castle believes he witnesses a murder in the apartment across the street.
When a body is discovered in an industrial cake mixer, the murder is traced to an Irish gang and a bar owner named Siobhan O'Doul (Cara Buono); Detective Ryan reveals a history with Siobhan from his time working under cover in narcotics.
Castle and Beckett tackle one of their creepiest cases yet when a woman appears to have been scared to death by a "haunted" DVD; Castle fears for his life after he accidentally watches the video.
After the FBI fails to get Alexis back, Castle takes matters into his own hands; Castle learns that his daughter's kidnapping may be a part of a larger and more sinister agenda.
A murder investigation reveals a plot to kidnap a wealthy businessman's daughter; FBI Agent Harris (Dylan Walsh) helps Castle and Beckett investigate.
The young rising star of "The Wives of Wall Street" is murdered; Castle tries to give Beckett a Valentine's Day gift, but his plans go awry.
Castle and Beckett discover evidence that links a young woman's murder to Sen. William H. Bracken (Jack Coleman) -- the man responsible for the murder of Beckett's mother; Beckett faces complications as she tries to finally bring Bracken to justice.
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