Taichi is a 20-year-old male college student. Yua is his first girlfriend and he really wants to lose his virginity with her. At that time, he is invited to her house. This is his chance to lose his virginity!! Just as he is thinking this, he is introduced to her older sister who lives with him. He gives up thinking this is impossible and goes to her house, but ends up meeting Mayu, who is prettier, cuter and bustier than his girlfriend, and is his type of woman. "Taichi, you're a virgin, right? The first time is the most important thing, so I'll be your practice partner..." she whispers to him.