Tsukasa Kozuki was raised as a boy by her police-officer parents to keep her safe in the Tokyo of 2020, a city where crime has swallowed the streets and women rarely walk them alone. After both parents die in a yakuza turf war, the family is left with nothing but debt, and Tsukasa takes work as bodyguard to her classmate Ryuji Shirogami, who became third-generation head of the Kuryuu-gumi at fourteen. Fighting through gang conflicts at Ryuji's side, she keeps up the act that she is a boy.