Kazuki Muto dies trying to save Tokiko Tsumura from a homunculus, an alchemical monster that hides inside people and feeds on them. Tokiko, a warrior of an order of alchemist soldiers, revives him by replacing his ruined heart with a kakugane, a device that takes the shape of a weapon matching its wielder. Now carrying a lance-form buso renkin, Kazuki joins Tokiko in hunting the homunculi and the masked alchemist who creates them.