The narrative of "Buso Renkin" follows Kazuki Muto, who saves Tokiko Tsumura and is killed by an alchemical monster known as a homunculus. Tokiko, an alchemist warrior, feels responsible and revives him by replacing his destroyed heart with a . The kakugane is an alchemical device which, when activated, takes a weapon form based on its user's personality, forming a buso renkinthe only thing that can destroy a homunculus. Kazuki creates his own buso renkin and joins Tokiko in the fight against the homunculi and their master, Koushaku Chouno. Chouno, who has renamed himself "Papillon", is killed by Kazuki, but is later resurrected by the L.X.E., a humanoid homunculi group led by Chouno's great-great-grandfather Bakushaku Chouno, now calling himself Dr. Butterfly. In various battles, Kazuki, Tokiko and Captain Bravo (their team leader) destroy most of the L.X.E.'s members. After learning that Dr. Butterfly believes him to be weak and useless, Papillon rebels against the L.X.E. and kills Dr. Butterfly.