Noboru Terao, a Japanese middle school student, shares an inseparable bond with classmate Mikako Nagamine, his intended high school companion. Their plans fracture when Mikako is conscripted by the United Nations Space Army to pilot a mecha in a far-flung interstellar war. As she voyages deeper into space, their sole connection—text messages—decays through transmission delays stretching from minutes to years.

Initially addressing Mikako formally by her surname, Noboru’s gradual shift to her first name mirrors his growing emotional dependence. Earthbound life marches forward: he enters high school alone, matures into adulthood, and wrestles with hope for her return against the gnawing void of separation. Meanwhile, relativistic effects freeze Mikako’s physical age in the depths of space.

Years later, Noboru enlists in the UN Space Force, joining a mission to retrieve Mikako’s spacecraft—a choice cementing his unwavering devotion despite decades of silence. An attempt to kindle romance with another girl collapses under the weight of his unresolved longing, laying bare his internal conflict.

At 24, Noboru receives an eight-year-old message from Mikako. Their mirrored monologues bridge time: his adult reflections clash with her preserved teenage voice, each clinging to sensory echoes of shared Earthly moments—summer clouds, rain’s patter, classroom air. This exchange etches their bond, unyielding to cosmic divides.

The manga adaptation unveils Noboru’s place on Mikako’s rescue roster, a discovery reigniting her hope. His arc transforms passive yearning into decisive action, embodying resilience against time’s erosion and the vastness isolating two hearts intertwined across light-years.