Castle Season 4 Episode 21
Watch Castle Season 4 Episode 21 Online
Episode Name: “Headhunters”
Episode Summary: 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.