Busou Renkin tells the story of Kazuki Mutou, an ordinary student who stumbles upon a battle between a girl named Tokiko Tsumura and a monster called a homunculus.
During the encounter, Kazuki unknowingly receives a powerful weapon called a Kurogane.
This weapon, manifested as a Busou Renkin, grants him special abilities.
Now, Kazuki is thrust into a hidden world where alchemically created monsters prey on humans.
He must learn to control his newfound powers and fight alongside Tokiko to protect humanity from the homunculi.
His life is forever changed as he struggles to balance his old life with his new responsibility as a warrior.