Watch Castle Season 4 Episode 21 online via TV Fanatic with over 7 options to watch the Castle S4E21 full episode. Affiliates with free and paid streaming include Amazon, iTunes, Vudu, YouTube Purchase, Google Play, TNT, and Verizon On Demand.
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It's a Firefly reunion on Castle this week, as Adam Baldwin guest stars in the fun role of Detective Ethan Slaughter.