Ritsuka Aoyagi, a twelve-year-old boy, is left with his mentally unstable mother as his sole kin after his brother Seimei's sudden death. Upon enrolling in a new school, Ritsuka encounters Agatsuma Soubi, who claims to have had a relationship with Seimei. Ritsuka eventually discovers that Soubi and Seimei were a fighting duo named "Fighter" and "Sacrifice" respectively. Soubi now becomes Ritsuka's "Fighter" since Seimei is no more. The organization, the "Seven Moons", is responsible for Seimei's demise. Ritsuka, joined by Soubi, sets out to uncover the truth behind his brother's death, although Soubi's assistance is sporadically futile.