Everyone associated with Beckett's mother's case is dead. Castle confronts his partner with that fact in this third season finale clip.
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Season three of Castle comes to an end with this episode. Titled "Knockout," it focuses on Beckett's mother's murder.
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Castle Season 3 Episode 24 Quotes

Unless you're a lot dumber than you look, you care about her too. Don't let her throw her life away.

Mr. Beckett

I've been going to that prison for the past four months to have a staring contest with the devil, and the devil just blinked.

Beckett