That summer day. Makoto Okano and his younger sister Hinata sit across from each other in the box seats of a local express heading to that town, underneath a blue sky that seems to stretch out to infinity. The town they are heading to is the very one that they were born in and were separated from long ago -- Kazune City. In the midst of the midsummer wind, bathed in the wind that rushes in from the train window, they head to that town, which they only have vague memories of. "Say, let's promise to meet again." "I'll keep playing this harmonica, okay? Because even if I can't reach you, the sound of the harmonica surely will." The harmonica, which was handed over together with the promise to that girl. And within the shaking cabin of the train, Makoto wakes up, dimly but surely recalling that memory ...