Get ready for an all-new episode of Castle, airing tonight Monday, April 16, 2012 at 10 p.m. ET.

So, what can we expect from tonight's much-anticipated Firefly reunion? Adam Baldwin guest stars as Detective Ethan Slaughter, a notoriously tough cop from the city's gang unit, who teams up with Castle and gives the mystery writer a dose of reality. Unlike Kate Beckett, who always has her partner's back, Det. Slaughter throws Castle right into the line of fire.

Will Castle run back to his former muse's loving arms, or will Slaughter's unorthodox methods pull him to the dark side for good?

Check out our sneak peeks and spoilers roundup for more juicy tidbits!

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


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