Ai-san and I are not real siblings. When she was a student, Ai was the child of her father's remarried partner. A year later, after his parents passed away, Ai became my parent. During the days we spent alone, I came to feel a special love for Ai-san, who was my only family member. Just when I vowed to make Ai-san happy in the future, my bad friend Shinji approached our family. "Your older sister is quite erotic and beautiful, isn't she?"