Throughout his youth, Juzo Murasaki is tormented and physically abused by his classmates. Lead by Tohru Akai, the abusers finally take this abuse way too far, pouring acid on Juzo's face in a science class. He is badly disfigured but his cold-blooded, unsympathetic attackers don't seem to mind.
Years later, Juzo gets a job working alongside his childhood nemesis on a construction site and learns that Tohru's abusive ways remain intact. Around the same time, Juzo undergoes a surreal and radical split. No longer visibly scarred, he manufactures a demonic, bullying alter-ego that appears to have inherited his formerly mangled face.
Known as No. 13, this violent figure appears only when Juzo's rage reaches the boiling point. Seeking brutal vengeance for all the indignities committed against Juzo in childhood, No. 13 targets Tohru and anyone else who gets on Juzo's nerves.