Kotarou is a brash young orphan in war-torn Japan, on the run from mysterious pursuers with Tobimaru, his faithful canine companion. Clueless as to why anyone would want him, much less imperial warriors from mainland China, a chance encounter with a strange wandering swordsman could not be more unwelcome in Kotarou's skeptical eyes – especially when the stranger has a secret past that has caused him to seal his blade. Forced by circumstance to work together for survival, the unlikely duo forge a tenuous friendship that is threatened all too soon when Kotarou's pursuers thrust the two into a dizzying whorl of clashing ambitions between men both big and small. Will the stranger manage to overcome his past and save them both from peril with his blade?
Akiyuki lives on post-war Sentan Island, always rushing between the demands of his separated parents and his friends at school. But his carefree days come abruptly to an end when his school bus suffers an explosion, the first attack to plunge Sentan back into war. Akiyuki lives to find himself turned into a Xam'd, a mysterious creature, by the suspected cause of the explosion. Engulfed by Xam'd, Akiyuki is eventually saved by a red-haired girl, Nakiami, only to find himself torn away from those he loves. Now he must work to control and understand the creature within him, all while searching for his family and friends in a world steeped in the secrets and chaos of war.