Castle, Season 3

[Castle explaining to Beckett why he had the gun at the murder scene]

Castle:
Maybe you missed the part where I said she was shot dead. When I heard the noises coming from the next room, I thought whoever killed her was coming back. So I picked up the gun to defend myself. It seemed like a very good idea at the time. That's when you, Esposito and Annie Oakley [gestures to the viewing room] come bursting through the door.

Montgomery:
Annie Oakley?

Ryan:
I kind of almost shot Castle. What? He had a gun.