Kazuna's family has a history of suffering from a disease that creates an uncontrollable vampire-like thirst for blood. Knowing none of this, Kazuna was sent to live with the Eda couple as a child. As the sickness begins to show signs of being present within himself, he comes into contact with his older sister, Chizuna, who has been suffering from it since early childhood. Knowing that the sickness incurs a heavy weight upon those that suffer from it, Chizuna tries to ease her brother's burden, to help him learn to control his urges, but... (Source: ANN)

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 after 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. (Source: ANN)