After he's attacked on the street at night by a roving motorcycle gang, timid bookkeeper Casey (Jesse Eisenberg) joins a neighborhood karate studio to learn how to protect himself. Under the watchful eye of a charismatic instructor, Sensei (Alessandro Nivola), and hardcore brown belt Anna (Imogen Poots), Casey gains a newfound sense of confidence for the first time in his life. But when he attends Sensei's mysterious night classes, he discovers a sinister world of fraternity, brutality and hyper-masculinity, presenting a journey that places him squarely in the sights of his enigmatic new mentor.