Kotone's husband, Takashi, is working away from home. Meanwhile, Takashi's divorced sister, Natsumi, is temporarily staying with Kotone and her husband at their home. Since both parents have already passed away, Kotone and Natsumi are living together. One night, Natsumi becomes excited when she hears moaning coming from Kotone's room. The next day, Natsumi's gaze as she looks at Kotone gradually becomes more mysterious...