Yuu Hasunuma has been passed between reluctant relatives since his parents died in a car accident, and the uncle who finally takes him in sets one condition: stay out of the storehouse. When that uncle ends up in the hospital, Yuu goes in anyway and finds a horned, hooved woman who offers to grant a wish in exchange for what he holds dearest. Never having had a family, he asks for an older sister, and she takes human form and the name Chiyo to live beside him.