12-year-old Aoyagi Ritsuka is a troubled child. He has no memories of his life until the age of ten. His excitable mother physically and verbally abuses him, demanding that he return the "real" Ritsuka to her. For the past two years, his older brother Seimei had been his devoted friend and protector: the only one who accepted Ritsuka's "new" self and who could stop their mother from abusing Ritsuka. In a talk the brothers had one day, Ritsuka learns Seimei's real name is Beloved, and although Seimei refuses to explain why he would say that, Ritsuka thinks the name fits. Soon after that conversation, Seimei was found dead under mysterious circumstances. A while later a college student named Soubi appears before Ritsuka, claiming to have been Seimei's friend. He tells Ritsuka that Ritsuka's real name is Loveless, and that like his older brother he is a sacrifice. Ritsuka has inherited Seimei's fighter, Soubi himself. Ritsuka finds himself drawn into a bizarre secret world where teams of two—fighter and sacrifice—engage in combat with another team using the fighter's verbal spells that manifest in local space and, for every round lost, create increasingly painful bindings on that team's sacrifice. As his involvement in this new life deepens, Ritsuka unearths many shocking facts about himself, Seimei and Soubi, plus the mysterious organization related to the combat, Septimal Moon. He resolves to discover the truths behind his brother's murder and his own unexplained amnesia.