Nanoka and Koshiro were separated at a young age when their parents divorced. Now, 10 years have passed, and Koshiro has already forgotten about his younger sister. Yet, Nanoka has decided to attend the nearby high school and come back to live with her father and brother. However, the two had met just before Nanoka came back to live together and without knowing each other's real identity, they went on a date, only to find out that they were actually siblings...