During her summer break at her grandparents' house, Tamai Shiina, a cheerful schoolgirl, meets a strange creature with whom she quickly develops a friendship. She names it "Hoshimaru: The Round Star." When she returns home for school, Shiina learns that she is not the only one who has befriended a peculiar creature like Hoshimaru. Unfortunately, this realization also makes her aware that not all creatures and their owners have good intentions like Hoshimaru.