Shiro is completely dependent on his mother, Rie, and has chosen a university within commuting distance from home. His sexual desires are solely focused on Rie, with no interest in anyone else. Meanwhile, Rie, a mature woman entering her prime, struggles with a sexless marriage and begins to develop a growing desire to build an exclusive relationship with Shiro. Now, a hot spring trip planned for just the two of them intensifies the tension. Though Rie harbors faint unease about crossing an irreversible line with her son, her emotions gradually begin to waver.