Castle and Beckett visit a crime scene in this clip from "Demons." It focuses on the death of a known... ghost wrangler.
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Castle tries to give Alexis some advice in this clip from "Demons." The speech doesn't go so well.
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Castle and Beckett enter the world of the supernatural this week. That makes Castle very happy, as you can see here.
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Castle Season 4 Episode 6 Quotes

If you say ghost, I'm sending you home.

Beckett

Alexis: Dad, are you using me as a shield against flesh eating zombies?
Castle: No, I'm just trying to give you a better view.