High school students heroic act turns into epic human vs. monster sword fight!

Buso Renkin is the story of teenager Kazuki Muto, who dies trying to save a girl who was being attacked by an eerie monster. The next morning, however, Kazuki is left wondering whether it was all a dream. Lo and behold, 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, and 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 the Alchemist Warriors, 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.

Kazuki's black kakugane, which powers his Buso Renkin weapon and keeps him alive, will soon advance to the point where it begins draining life from others around him. Teaming up with his partner Tokiko, Kazuki leaves on a mission to find out more about the black kakugane in hopes of keeping this from happening. Meanwhile they encounter an elite group of Alchemist Warriors that have been assigned to hunt them down and kill them!