Headhunters Castle : Season 4 Episode 21
TV-14 ABC 43m int(0)
Aired: April 16th, 2012 @ 9:00 PM EST on ABC
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