On an idyllic late-summer day in Saddlestring, Wyoming, game warden Joe Pickett is fly-fishing with his two daughters when he stumbles upon the mutilated body of a moose. Shaken by the sight, Joe starts to investigate what he hopes is an isolated incident. Days later, the discovery of a small herd of mutilated cattle makes him realize this is something much more terrifying than he could have imagined.