An astronaut is murdered in a Mars simulator. This one has to end up in one of Castle's books. It's simply too good not to use.
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Castle and Beckett don spacesuits and enter a Mars simulation to figure out how an astronaut ended up dead on Castle. "The Wrong Stuff" is the 16th episode of the show's seventh season.
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We really hope that this is the last time Castle and Beckett will have to face off against 3XK.
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Can Castle and Beckett find a way to take down 3XK even though he's stolen all of the evidence they previously had against him?
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Will Capt. Gates help Castle get back to the 12th permanently if he helps close this case?
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Castle tried to tell everyone that this guy was still alive, even after he fell off the bridge into the river.
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And before you know it Rick Castle's arch nemesis is back and it appears he's enjoying egging Castle and Beckett on once again.
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Annie Wersching returns for this two-parter to once again play Dr. Kelly Niemann, 3XK's partner in crime.
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Castle Quotes

Sometimes the hardest things in life are the things most worth doing.

Castle

Beckett: (Clears throat) Ahem. What's the deal with men and boobs, anyway?
Castle: Biological. We can't help it.
Beckett: But doesn't it bother you that they're so obviously not real?
Castle: (Pauses) Santa's not real. We still love opening his presents.