The descendants of a revered Okinawan martial arts master set down opposing life paths following a childhood trauma that leaves them both deeply scarred. Ayaka and Natsuki were on the path to becoming masters like their father when a man killed their father, abducted Natsuki, and stole the family's prized black belt. Flash forward a few years, and Natsuki has been transformed into a ruthless assassin by a malevolent sect, and Ayaka has grown into a humble high school student whose martial arts ambitions were lost with her only sibling.