Seigetsu Academy, once an all-boys school, is now a co-ed institution that focuses on teaching astronomy. In this context, Tsukiko Yahisa enrolls as the academy's first and only female student. Her childhood best friends, Kanata Nanami and Suzuya Touzuki, stay by her side to shield her from the advances of the student body. As Tsukiko learns to navigate her new environment, she encounters 13 distinct boys based on the Western zodiac signs. Eventually, Tsukiko's involvement with these boys pulls her deeper into star-crossed routes that will entangle her in numerous affairs.