Buso Renkinis the story of teenager Kazuki Muto who dies trying to save a girl who was being attacked by an eerie monster. The following morning, Kazuki wakes wondering whether it was all a dream. But the girl, the monster, and his death are all real! The girl, Tokiko Tsumura, was actually trying to slay the homunculus (a beast that can take the form of humans, but whose main source of food is people), but Kazuki got in her way. To revive Kazuki, Tokiko replaces his heart with a "kakugane," an alchemic device that allows him to summon a lance with which to fight the monsters. It turns out that Tokiko is a member of Renkin Kenshi(Alchemist Soldiers), an organization sworn to protect the world from the diabolical creatures. Soon, Kazuki joins Tokiko in her quest to terminate the sinister being that creates and controls the homunculus. Never underestimate the power of underwear! With no battle gear to speak of besides his butterfly mask and skivvies, Papillon Mask survives a bitter duel against our boy Kazuki. Meanwhile, Papillon Mask's great-great-grandfather Dr Butterfly promises him the true powers of a humanoid homunculus, which includes the ability to use a Buso Renkin. Will Kazuki and Tokiko be able to fend off their very powerful - and eccentric - nemesis?