Tsukiko Yahisa is the first female student to enroll at Seigetsu Academy, a former all-boys school specializing in astronomy. She is closely watched over by her childhood friends, Kanata Nanami and Suzuya Tohzuki, who often protect her from the male student body. Throughout the games, she befriends other young men whose personalities are based on the Western zodiac constellations.
In the first game, Starry☆Sky~in Spring~, not long after Tsukiko transfers to Seigetsu Academy, a half-French boy named Yoh Tomoe follows suit to pursue her. This action stirs up emotions from Kanata and Suzuya, both of whom had long loved Tsukiko.