Sonnet, a cyborg/esper with a turbulent past, was trained to become the ultimate warrior and the world's most powerful weapon. Her mission is to monitor Komatsuzaki Lan in Japan, who is believed to be manipulated by the merciless esper Akai Kiba (Crimson Fang). Despite being reserved, Lan realizes that she is exceptional and begins to display the symptoms of the Crimson Fang after encountering Sonnet. As Sonnet battles with Lan, she begins to rediscover her empathy, while Lan struggles to hold onto her humanity and manage the Crimson Fang.