While the team searches for a way to disarm the explosive, Castle distracts Beckett by arguing with her about who fell for whom first.
In the Season Finale, an interview with a high-profile federal law enforcement agency forces Beckett to reconsider what she wants out of her job — and out of her relationship with Castle. Meanwhile, the body of a young woman is found floating inside the water tank of a skid row flop house, but as the team investigate, they discover that her final resting place is just one of many strange things about her death.
When Homeland Security inexplicably seals the crime scene of a car bombing, Castle and Beckett find themselves with two mysteries on their hands: who murdered the victim, a government whistleblower, and why are federal agents trying to take over the case? The plot thickens when they discover that the victim wasn’t killed by a car bomb but by a missile from a military drone.
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.
When a young woman appears to have literally been scared to death three days after watching a "haunted" DVD, Castle and Beckett delve into one of their creepiest cases yet. To make matters worse, Castle becomes convinced that he too is marked for death when he accidentally watches the cursed video. Master of horror Wes Craven makes a special quest appearance.
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.
When a man is found dead in an industrial cake mixer, Castle and Beckett trace the murder to an Irish gang in Staten Island and a beautiful bar owner named Siobhan O'Doul (guest star Cara Buono, "Mad Men"). Much to everyone's surprise, Detective Ryan has a complicated history with Siobhan and the Staten Island crime family from his days working undercover in narcotics. When the case hits a dead end, Ryan must go back undercover to expose the killer, complicating his relationship with his wife.
In the first of a two-episode story arc, Castle & Beckett begin a murder investigation that also exposes a plot to kidnap the daughter of a wealthy Middle Eastern businessman. Dylan Walsh ("Nip/Tuck") guest stars as FBI Agent Harris, an even-keeled professional brought in to help find the kidnappers.
When Castle and Beckett find giant footprints at the site of a vicious murder, the team is forced to confront the unlikely theory that Bigfoot is the killer. But as they trek into the world of primate research, big game hunting and mythological creatures, they realize that finding the man (or beast) responsible may put them on the endangered species list.
When Beckett is assigned to protect Erik Vaughn, a charming billionaire entrepreneur whose life is in danger, she's forced to reflect on her relationship with Castle which has begun to plateau. Meanwhile, consumed with jealousy, Castle goes to extraordinary lengths to solve the case and separate the two of them.