After losing his mother, Tatsuya has been unable to get along with Saeko, his father's new wife, for over a year. With his father now overseas, Tatsuya finds himself acting coldly toward Saeko despite his inner struggles. Yet Saeko remains cheerful and kind, unaffected by his attitude. One day, Tatsuya accidentally discovers Saeko masturbating. To his shock, he sees her fingers penetrating her anus. The vulgar sight of her exposed back door—so different from her usual gentle and modest demeanor—leaves a deep impression on him. From that moment on, his feelings toward her begin to slowly change.