Set sometime in the future, the story begins with Kei and Ai fleeing for their lives. Ai is a young girl harboring a terrible secret that could destroy the current world. The world is now populated by Headmeters, people with psychic powers whose strength is displayed on their foreheads during combat. However, Kei was an experimental subject, and his strength never exceeded level 5, while others surpass his ability effortlessly. While escaping, in desperate moments, Kei could increase his ability to infinity when Ai is in trouble. Those pursuing her hold a deep secret, aided by a small, dwarf-like man living in a robotic suit. Eventually, Ai is captured, and Kei, a Sherlock Holmes-style detective, along with another Headmeter who switched to the good side after Kei unleashed his horrible powers through Ai, must go rescue her. Together, they must save Ai and the secret she holds. What is Ai's secret? Who is she?