Nathan Fillion Nathan Fillion

On Monday's Castle (10/9c, ABC), Castle (Nathan Fillion) and Beckett (Stana Katic) investigate the murder of a Japanese ballet dancer. But just when they stumble into the killer's lair, Castle is attacked by a shadowy figure who steals what the detective duo believes to be the murder weapon.  But after seeing the perpetrator with his own two eyes, Castle is convinced the murderer is a ninja!

Castle boss: "The gloves are off" between Beckett and Bracken

"Or he is just an athletic person with a hooded track suit," the always skeptical Beckett interjects during the pair's briefing with Capt. Gates (Penny Johnson Jerald) in the exclusive sneak peek video below.

So is Castle finally right? Or is this just another outlandish story from his overactive imagination? Press play below to hear all of Castle's supporting evidence.

Castle airs Mondays at 10/9c on ABC.