4.21 – Headhunters

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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.

4.16 – Linchpin

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In the explosive conclusion of a two-part story arc that began last week with “Pandora,” Castle & Beckett race to find the “Linchpin” in order to stop the first domino in a chain of events which will plunge the United States into war.

4.09 – Kill Shot

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A sniper on a killing spree is terrorizing New York City. With the clock ticking down to the next murder and nothing to go on except a killer with no apparent motive but to instill terror, this could prove the team’s toughest case yet. The hunt is made even more complicated when Beckett begins to experience increasingly strong moments of PTSD – moments she tries hard to hide from Castle and the detectives.

4.15 – Pandora

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In the first episode of a two-part “Castle” event, 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 (guest star Jennifer Beals, “The Chicago Code,” “Flashdance”), a smart, sexy CIA operative who has a complicated past with Castle. Josh Stamberg, (“Drop Dead Diva”) also guest stars as Martin Danberg, an intelligence officer.

4.17 – Once Upon a Crime

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When a woman dressed as Red Riding Hood is found dead in the woods with claw marks covering her body, and soon after a murdered Snow White is discovered with a poison apple in her hand, Castle & Beckett find themselves on a Grimm hunt to catch a fairytale killer before he kills again. But nothing is as it seems in a case filled with deceit, betrayal, secrets and murder.

4.18 – A Dance with Death

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When Odette Morton, a contestant on the hit competition show “A Night of Dance,” is found murdered in her dressing room, Castle and Beckett must navigate the case’s many twists and twirls without stepping on each other’s toes. But as they delve into Odette’s past, they discover that this reality starlet was harboring a shocking secret that may have led to her death.

4.22 Undead Again

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When Castle and Beckett investigate the murder of a man with human bite marks on his body, Castle’s wild theories start flying. But none are as wild as what their only witness insists happened – a Zombie attack. As the evidence pointing towards an undead assailant piles up, the team plunges into New York’s Zombie subculture to find the killer and bring him in — dead, undead or alive.

4.11 – Till Death Do Us Part

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When a lothario is killed, Castle and Beckett suspect that the victim’s mysterious double life holds the key to his murder. Their investigation leads to a shocking twist that threatens to disrupt Ryan’s wedding with Jenny.

4.04 – Kick the Ballistics

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While investigating the mysterious shooting death of a young woman, Castle and Beckett face a cop’s worst nightmare when they discover that the gun used in the killing is Det. Ryan’s old service weapon – the one that was stolen by the serial killer, 3XK. Now with the clock ticking, the team must work to solve the murder, track down 3XK and retrieve Ryan’s gun before the killer strikes again.

4.19 – 47 Seconds

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When a bomb explodes at a protest rally killing five people, Castle & 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. Christine Woods (“Perfect Couples”) and Tim Guinee (“The Good Wife”) guest star.