The Nohara family was enjoying a trip in Okinawa, when a bomb, dropped by aliens "Ketsudake Seijin" ("The Only Buttocks Aliens"), gets stuck to the rear end of Shiro by accident. The incident is found out by the UNTI (Unidentified Nature Team Inspection), an organization that monitors the universe, and the UNTI starts a recovery operation. Shinnosuke learns that the bomb is powerful enough to blow up the whole Earth and that it cannot be taken off Shiro by any means.