One day, on a whim, 20-year-old Minato dresses up in her friend's cute high school uniform and briefly goes out into the city. To her horror, she ends up running into her stepbrother, Tooru, who is actually only months younger than her. Since he is cute and a notorious player, Minato has had to deal with constant jealousy and suspicion from other girls throughout their school days. Amazingly, she manages to convince Tooru that she is actually someone who just happens to look a lot like his sister. He is so convinced that he starts trying to date her and, for various reasons, she goes along with it. What started as a simple joke is now evolving into the biggest lie Minato has ever told!