Mondays 10:00 PM on ABC
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Looking to watch Castle Season 4 Episode 21 online? TV Fanatic is your destination for every paid and free streaming option of the Castle S4E21 full episode from 2 affiliates.

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Episode Details

Castle Season 4 Episode 21: "Headhunters"

It's a Firefly reunion on Castle this week, as Adam Baldwin guest stars in the fun role of Detective Ethan Slaughter.