Come and be my assistant." Kimizuka Kimihiko, a boy with a physique that is easily involved in incidents, becomes the assistant of Hiesta, an angelic and beautiful girl who claims to be a detective, in an airplane at an altitude of 10,000 meters. In order to fight enemies all over the world, the two flew around the world for three years and embarked on a dazzling adventure. ——Soon after, the detective died. A year has passed since the turbulent days. Kimizuka, who has become a third-year high school student, has also indulged in the warm water called daily life and lives an extremely ordinary student life. A client appears next to Kimizuka. "Are you a famous detective?" With the opportunity of meeting Nagisa Natsunagi, a girl of the same grade, the grand story connecting the past and the present begins again——