Twelve-year old Aoyagi Ritsuka lost his only kin when his brother, Aoyagi Seimei, was killed under mysterious circumstances. One day, he meets Agatsuma Soubi, who claims to know his brother. It turns out that Soubi and Seimei used to be a fighting pair, whereby Soubi was the Fighter 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 Nanatsu no Tsuki, Ritsuka decides to join forces with Soubi and investigate the truth behind his brother's death.