On his first day at his new school, 12-year old Ritsuka Aoyagi meets a mysterious 20-year old man named Soubi Agatsuma. He claims to be a good friend of Ritsuka's brother, Seimei, who was recently murdered, and the suspicious organization called 'Septimal Moon' (ななつの月, Nanatsu no tsuki?) may be responsible for the death. As Ritsuka quickly finds out, Seimei and Soubi acted as a pair involved in spell battles. Now Soubi is Ritsuka's 'sentoki', or fighter, and Ritsuka is his 'sacrifice'. Together, they challenge the organization to find out the truth behind Seimei's death and the reason for Ritsuka's amnesia, but end up forming an intimate bond with each other as they unravel the mystery.