Found as a newborn beside the cursed katana Shiryū, Gai is taken in by the swordsmith Amon Ogata and raised as his apprentice. When a ceremony meant to subdue the blade goes wrong and costs Gai his right arm, Amon forges Shiryū itself into a prosthetic limb, binding boy and sword together for good. The manga follows Gai through the fights that come with that fusion, alongside Amon's daughter Sayaka and Seiya Ichijou, the strongest of the sword-bearing Chrysalis, weighing what the blade costs him against what it lets him protect.