Mako, a young girl, comes to her parents' house to help with the housework after her mother is hospitalized due to illness. While she is chatting with her father, her younger brother returns home, but he seems a little distant. Curious, Mako peeks into her room and finds out that her brother has bought a masturbator. Seeing that he doesn't know how to use it, Mako realizes that he is a virgin and starts to tease him, but he pushes her down with the force of his adolescence... From that day on, she is at the mercy of her younger brother's sexual desires...