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.

4.14 – The Blue Butterfly

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When Castle and Beckett investigate the killing of a treasure hunter, they discover the case is linked to a mysterious homicide from 1947 involving a hard-boiled private detective. Castle realizes the only way to solve the present-day murder is to solve the murder from the past. The 1947 case comes to life through stylized flashbacks, featuring Castle as the private eye and Beckett as a femme fatale.

4.05 – Eye of the Beholder

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Castle & Beckett work a murder case around the theft of a valuable sculpture. Kristin Lehman (“The Killing”) guest stars as Serena Kaye, a smart, sexy insurance investigator, who joins Castle & Beckett in the investigation, but she also has her eyes set on Castle, all the while hiding a very important secret from the team. Jessica Tuck (“True Blood”) also guest stars as Joy McHugh, a wealthy museum benefactor who becomes a suspect in the murder investigation.

4.23 – Always

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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, on the Season Finale of “Castle.”

4.01 – Rise

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In a riveting opening that picks up just moments after last season’s climactic finale, Detective Beckett (Stana Katic) fights for her life as Castle (Nathan Fillion), plagued by guilt over his role in the events, struggles to uncover who’s behind her brutal shooting. Meanwhile, back at the precinct, Ryan (Seamus Dever) and Esposito (John Huertas) have to adjust, as Victoria “Iron” Gates (Penny Johnson Jerald, “24”) becomes the precinct’s new uncompromising Captain.

4.03 – Head Case

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Gunshots, a fleeing van and a massive blood pool all point to murder, but when Castle & Beckett arrive at the crime scene, they discover that it lacks something – the victim! The investigation leads to the cutting-edge world of life-extension science, a cunning high-tech researcher (guest star William Atherton), and a sleazy porn mogul who invested in a modern-day “fountain of youth.” Meanwhile, Alexis finds out whether she got into Stanford.

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.

4.08 – Heartbreak Hotel

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During the investigation of the murder of an Atlantic City casino owner at an abandoned warehouse, strong leads emerge for the team in both New York and Atlantic City. As Beckett and Capt. Gates work the New York leads together, Castle naturally chooses to join the detectives for the road trip to “America’s Playground,” hoping to help solve the case while throwing an impromptu bachelor party for Ryan.

4.02 – Heroes & Villains

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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?

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.