12-year old Aoyagi Ritsuka, on his first day of 6th grade meets Agatsuma Soubi, who claims to be his deceased older brother's friend. They discover that Ritsuka's older brother, Seimei, was murdered by the mysterious organization, called the 'Seven Moons' (ななつの月 nanatsu no tsuki). As the 'Sacrifice' and with Soubi as the 'Fighter,' they make use of the fighting style that uses language to cast spells. They challenge the organization to find out the truth behind Seimei's death and the reason for Ritsuka's amnesia.