Tsukiko Yahisa arrives at Seigetsu Academy as its first ever female student. The school has always been all-boys, its focus on astronomy drawing a specific kind of student. Her presence changes the atmosphere. Two childhood friends — Kanata Nanami and Suzuya Tohzuki — watch over her. Through the school year she comes to know the young men around her, each of whose personalities are tied to a sign of the Western zodiac.