Twelve-year old Aoyagi Ritsuka lost his only kin when his brother, Aoyagi Seimei, was killed under mysterious circumstances. One day, he meets Wagatsuma Soubi, who claims to know his brother. It turns out that Soubi and Seimei used to be a fighting pair, whereby Soubi was the "Fighting Machine" and Seimei was the "Sacrifice". Now that Seimei is gone, Soubi 'belongs' to Ritsuka who will become his new "Sacrifice". After learning that Seimei was killed by an organisation known as the "Seventh Moon", Ritsuka decides to join forces with Soubi and investigate the truth behind his brother's death.