Tsukiko Yahisa is the first female student to enroll at Seigatsu 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 Seigatsu 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.
In the second game, Starry☆Sky~in Summer~, Tsukiko has joined the academy's archery club. As she practices for the summer competition, she simultaneously solves the problems between her teammates (Homare Kanakubo, Ryunosuke Miyaji, and Azusa Kinose).
In the third game, Starry☆Sky~in Autumn~, Tsukiko becomes involved with three teachers at her school (Iku Mizushima, Naoshi Haruki, and Kotarou Hoshizuki).
The last game, Starry☆Sky~in Winter~, details Tsukiko's involvement with the student council, which includes Tsubasa Amaha, Kazuki Shiranui, and Hayato Aozora.

So basically, the anime focus on Tuskiko's interaction with the 13 guys.