Due to their father's work situation, Natsumi and her younger brother Jiro live alone together. Natsumi is a 28-year-old office lady in the prime of her womanhood, finding joy in drinking canned beer after her bath. However, she's poor at cooking and generally clumsy, always relying on her younger brother Jiro. Unbothered by this dependence, she becomes infatuated with her boss at work, her emotions running high. But this routine suddenly comes to an end when Natsumi suffers a heartbreaking rejection. That night, returning home drunk, she pins down Jiro—who was waiting for her—and confesses that he's still a virgin. Overcome by lustful desires, she begins to seduce her own brother...