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 kakugane (核鉄).  The kakugane is an alchemical device which, when activated, takes a weapon form based on its user's personality, forming a buso renkin—the only thing that can destroy a homunculus.