Five years ago, after discovering her husband's affair, Mariko was comforted by an awkward kiss from her adult son's friend Hajime. On the day of his graduation, she overcame her conflict and guilt, returned his feelings, and began a sexual relationship with him. They have secretly remained friends with benefits behind her husband and son's backs ever since. Now Hajime has a girlfriend, forcing Mariko to decide whether their five-year affair can continue.