During her summer holiday at her grandparents house Tamai Shiina, a young and cheerful schoolgirl, meets a strange looking creature. They befriend each other and Shiina names it "Hoshimaru - The Round Star". When Shiina returns home efter the summer to go back to school she starts meeting other kids that also have befriended a strange creature like Hoshimaru. But she soon finds out that not all these creatures and their masters are as friendly as Hoshimaru.