CASTLE Season 4 Episode 21 Headhunters Promo : ”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.
