One morning, the Detective Boys are on their way to an abandoned building lot which Professor Agasa has managed to loan from a wealthy acquaintance, in order to practice their soccer skills for an upcoming school match. The lot is located next to an eight-story building, and was formerly the site of a mansion which is already torn down, with only a family-sized pool and a shed remaining. However, when they arrive at the lot, they find the grounds crowded by the police: A body has been found in the pool.