On his first day at his new school, twelve-year-old Ritsuka Aoyagi meets a mysterious twenty-year-old male named Soubi Agatsuma. Soubi claims to be a good friend of Ritsuka’s brother, Seimei, who was murdered when Ritsuka was ten years old. Upon the inspection of Seimei’s abandoned computer files, Ritsuka discovers that an organization called ‘Septimal Moon’ (Nana no tsuki) was responsible for Seimei’s death.
As Ritsuka quickly discovers, Seimei and Soubi acted as a pair involved in spell battles invoked by carefully selected words. Now Soubi is Ritsuka’s ‘sentouki’, or Fighter Unit, and Ritsuka is his ‘Sacrifice’. Together, they challenge Septimal Moon to find out the truth behind Seimei’s murder and the reason for Ritsuka’s amnesia, and form an intimate bond as they unravel the mystery.