In the futuristic setting of 2046, Noboru Terao and Mikako Nagamine, close friends on the brink of graduating junior high, face a sudden alien invasion. Mikako gets chosen for a vital UN military space mission to counterattack the extraterrestrial threat, setting her on a path far from Noboru. To stay in touch, they rely on exchanging emails via their cell phones. However, as the space expedition progresses beyond Earth, the time delay in their message transmissions escalates significantly. 'Hoshi no Koe' delves into the emotional bond of this duo as they grapple with maintaining their connection across the expanding cosmic void.