Once thought to be extinct, ninjas have resurfaced in modern-day Japan after the Pacific War. Rumored to number two hundred thousand, these lethal assassins are now assigned to covert operations, infiltrating a substantial part of the country's public and private institutions in the process. One of these ninjas is Kurou Kumogakure, who frequently spends his days in a dilapidated apartment, lacking any drive to do much of anything. Some might view him as a useless drifter who does nothing but steal beer from his neighbor. Still, this 17-year-old underachiever might actually be more dangerous than his appearance implies. Kurou's concealed potential is tested when he receives a new mission to infiltrate a local high school, but there is more to this assignment than he could ever anticipate.