Yuu lost his parents in a car accident at age five and has spent the years since being passed between relatives, never finding a real home. Lonely and wanting companionship, he meets the demon Shub-Niggurath, the Black Goat of the Woods with a Thousand Young, who calls herself Chiyo and offers him a single wish. His answer: he wants her to become his big sister. The OVA, adapted from Iida Pochi's manga, mixes romance, ecchi, supernatural horror and slice of life.