Aired: April 17, 2012 | Season: 4 | Episode: 21

Overview
Runtime: 44m
Directed by: John Terlesky
Written by: Alexi Hawley
Guest stars: Adam Baldwin, Michael Dorn, Michael McGrady
Description
With tensions rising between Castle and Beckett, Castle finds himself a new partner -- Detective Ethan Slaughter, 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.