High School student Kazuki Muto thinks he's saving a girl from a monster, but it turns out that he's the one who needs saving! Kazuki is killed by a homunculus, a malevolent creature that feeds on humans. The girl, Tokiko Tsumara, revives him by replacing his heart with an alchemical device called a kakugane. With this device he can create his own Buso Renkin, an alcehmical weapon in the form of a huge lance, capable of destroying homunculi.