In 2046, Noboru Terao and Mikako Nagamine are close friends nearing their junior high school graduation. However, following an alien assault on humanity, Mikako is chosen for a UN military space mission intended to retaliate against the invaders. Now separated, Noboru and Mikako strive to maintain contact by exchanging emails via their cell phones. As the expedition travels further from Earth, the delay in transmitting and receiving their messages grows significantly. Hoshi no Koe centers on the relationship between this pair as they endeavor to remain connected despite the widening gap between them.