SpoilerTV Image Archive | Gallery | Castle | season-4 | promotional-episode-photos | Episode 4.21 - Headhunters | 127821_9355

CASTLE - "Headhunters" - 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, on "Castle," MONDAY, APRIL 16 (10:01-11:00 p.m., ET), on the ABC Television Network. (ABC/RON TOM) NATHAN FILLION

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