Sonnet is a cyborg esper raised in harsh conditions and trained to become the ultimate warrior, the world’s most powerful weapon. She is dispatched to Japan to monitor Komatsuzaki Lan, a quiet girl who believes she is distinct from everyone else and is thought to be controlled by the rage of the esper Akai Kiba, also known as Crimson Fang. After coming into contact with Sonnet, Lan begins to show signs of Crimson Fang. As they clash, Sonnet gradually regains her humanity, while Lan must fight to retain her own humanity and master the Crimson Fang.