Bringing her boyfriend home might be one of the unforgettable lessons for the younger sister, especially with her promiscuous older sister Yoshizawa still around. With their parents away, only the older sister was at home, so the younger sister naively brought her boyfriend over, hoping to strengthen their relationship. However, instead of growing closer, she faced a high risk of losing her boyfriend. The very person she trusted the most, her older sister, was the one who carried it out without mercy. With her beautiful and alluring appearance, it was easy for the older sister to attract the younger sister's boyfriend, who was also incredibly lustful, ready to do anything upon seeing a pretty girl. Just a moment of distraction from the younger sister was enough for them to seize the time to conspire and engage in their illicit affair. The incident has happened so many times that when her younger sister directly caught the two of them in the act, she couldn't do anything about it; they even chased her away and continued to satisfy their physical needs.