Shiro is a total mama's boy, and went to a university within commuting distance of his home. Furthermore, he Kokoro feels sexual interest in his mother, Rie. Rie, who is at the peak of her sexuality, would rather have a sexual relationship with Shiro than with her sexless husband. Today, Rie and Shiro are going on a hot spring trip alone. Rie is a little worried that she might end up with Shiro on this trip...