A young college student named Taiichi, aged 20, was eager to lose his virginity with his first girlfriend Yua. One day, he was invited to her house. This could be his chance to graduate from virginity! However, when he arrived at her home, he met not just Yua but also her older sister Mayu, who was stunningly beautiful, adorable, busty, and irresistible. She offered herself up as a practice partner, saying, "Taiichi-kun, you're still a virgin, right? Since it's important to get started, let me help you out..."