Check out this clip from the episode "The Final Nail." It features the title character considering two terrible possibilities.
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This isn't anyone else, Beckett! Castle argues with his quasi partner in this scene because she suspects his pal of murder.
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Castle is placed in an awkward spot on "The Final Nail." His friend is suspected of killing his wife.
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What is Castle doing here? He's old friends with the murder suspect on "The Final Nail."
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Jason Wiles guest stars this week as a friend of Castle's. Also, as a murder suspect on the episode "The Final Nail."
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Castle Season 3 Episode 15 Quotes

You have a great talent, no one had said anything like that to me before.

Castle

Shut the front door.

Castle