Castle pays Beckett a visit in this clip from the episode "Headhunters." Which makes her happy, until she learns why.
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Get a look here at Adam Baldwin on Castle. His detective comes between Rick and Kate a bit, as previewed in this clip from "Headhunters."
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Castle Season 4 Episode 21 Quotes

Forget severed heads, I want to meet that guy!

Castle

Martha: Playing with dolls, are we?
Castle: They are action figures.