Noboru Terao and Mikako Nagamine are close friends who will soon graduate from junior high school in the year 2046. However, following an alien attack on humanity, Mikako is chosen for a UN military space expedition to retaliate against the aliens. Separated, Noboru and Mikako try to communicate by sending emails to each other using their cell phones. However, as the expedition travels further away from Earth, the time it takes to transmit and receive messages increases significantly. Hoshi no Koe is about a couple's relationship as they try to stay connected despite growing distance between them.