Naoko is visiting Nagasaki's Nakiri Island to receive treatment for her asthma when she meets Yuusei, a boy who loves to run. One day, she falls into the sea, and Yuusei's father goes in to help. He saves her, but dies in the process, traumatizing both kids. 6 years later, Naoko meets Yuusei again, this time in Tokyo. Where he's become a brilliant runner.