Makoto Okano and his younger sister Hinata ride a local express train back to Kazune City — the town they were born in and left behind long ago. Beneath the midsummer wind pouring through the train window, a memory stirs: a promise made to a girl, and a harmonica handed over alongside it. "I'll keep playing this harmonica, okay? Because even if I can't reach you, the sound surely will." Inside the swaying train cabin, Makoto slowly, dimly wakes up — and remembers.