After a roving motorcycle gang attacks him on the street, timid bookkeeper Casey joins a neighborhood karate studio to learn how to protect himself. Under the watchful eye of a charismatic instructor, Sensei, and hardcore brown belt Anna, Casey gains a newfound sense of confidence. But when he attends Sensei's mysterious night classes, he discovers a sinister world of brutality and hyper-masculinity, presenting a journey that places him squarely in the sights of his enigmatic new mentor.