12-year-old Aoyagi Ritsuka is a troubled child. He has no memories of his life until the age of ten - causing his excitable and verbally-abusive mother to constantly berate him, demanding that he leave and return the 'real' Ritsuka to her. For the past two years, his older brother Seimei had been his devoted friend and protector - the only person who accepted Ritsuka's 'new' self. Recently, however, Seimei was found dead under mysterious circumstances.

Soon afterward, a man named Soubi appears before Ritsuka, claiming to be Seimei's 'friend'. He informs Ritsuka that his name is "Loveless", and that he is a 'sacrifice' - while Soubi is his 'fighter'. Since their first meeting, Ritsuka finds himself drawn into a bizarre secret world where teams of two; the 'fighter' and 'sacrifice', combat each other using mystical verbal spells.

As his involvement in this new life deepens, Ritsuka unearths many shocking and disturbing facts about himself, Seimei and Soubi; thus, he resolves to discover the truth behind his brother's murder and his own inexplicable amnesia.