Season 4 Episode 21 episode 21

'Castle' Season 4 Episode 21 "Headhunters" Promo

'Castle' Season 4 Episode 21 "Headhunters" Promo

Feeling that he is no longer able to work with Beckett, Castle finds a new partner in Detective Ethan Slaughter (guest star Adam Baldwin). It's a mini "Firefly" reunion! Check out the promo for Castle season 4 episode 21 "Headhunters" airing on April 16th.