Busou Renkin tells the story of teenager Kazuki Mutou, who dies while attempting to save a girl being attacked by a creepy monster. The following morning, however, Kazuki is left wondering if it was all just a dream. To his surprise, the girl, the monster, and his death were all real! The girl, Tokiko Tsumura, was actually trying to slay a homunculus (a beast that can take human form but feeds primarily on 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 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 these diabolical creatures. Soon, Kazuki joins Tokiko in her quest to terminate the sinister being that creates and controls the homunculus.