Sonnet is a cyborg/esper from a harsh background and now trained to be the ultimate warrior and most powerful weapon in the world. She is sent to Japan to watch Komatsuzaki Lan, who is thought to be controlled by the rage of the esper Akai Kiba (Crimson Fang). Lan is a quiet girl who knows she's different from everybody else and starts to show signs of Crimson Fang after coming into with Sonnet. In the course of fighting with Lan, Sonnet starts to rediscover her humanity. At the same time Lan has to fight to retain her humanity and control the Crimson Fang.