Ushinawareta Mirai o Motomete

Uchihama Academy has seen an increase in enrollment in recent years and as a result, a new school building will be constructed. The story takes place during the final school festival in the old school. All the club leaders decide to give it their all to make this the best school festival. Sou is a member of the notorious astronomy club, which basically solves the various issues in the student council's place.
On one day, he stays in the school late at evening because he got slightly injured by the judo club and is healing his wounds in the infirmary. Suddenly, the building shakes. Shocked, he runs up the stairs to where the shaking seems to have centered, and there he encounters a mysterious naked girl, whom at first appears to know him. On the next day this girl, Yui, seeks out the astronomy club, expressing her desire to join.