Returning home to give birth, a daughter and her husband come to stay at her parents' house. Among them, the son-in-law begins helping his mother-in-law, Mai Ko Kashiwa, with household chores in place of his pregnant wife. While attempting an unfamiliar task, he injures his finger, prompting Mai Ko to instinctively put his finger in her mouth to stop the bleeding. The son-in-law is stunned by her actions and becomes deeply aware of his own feelings. That night, unable to sleep, he stares at his finger in the kitchen when Mai Ko appears in light clothing. Overcome by desire at the sight of her, he becomes aroused. Recognizing his reaction, Mai Ko moves closer, lowering her face toward his crotch, and their relationship takes a forbidden turn.