Spoiler Photos of Castle Season 4, Episode 21: "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."

Original Air Date: Castle Season 4, Episode 21: "Headhunters" airs on Monday, April 16, 2012 at 10 p.m. ET on ABC.