Kinji Tohyama is a student at Tokyo Butei High school, where special detectives known as Butei are trained to become mercenaries. Whilst riding his bike to school after missing the bus, Kinji is chased by a remoted controlled segway armed by an uzi, telling him that a bomb has been attached to his bike which will explode if he slows down, also threatening to shoot him if he jumps off. Kinji is saved a mysterious twintailed girl named Aria H Kanzaki. As the two end up pinned down in a gym storage locker as more segways appear, Aria fends them off, sticking her chest right in Kinji's face…