Mikako Nagamine, a junior high school student, is thrust into an extraordinary destiny when the Tarsians—aliens originating from Mars’ Tharsis region—launch an attack on Earth. Recruited by the UN Space Army, she becomes a Special Agent piloting a Tracer, a towering robotic mecha deployed from the spacecraft carrier *Lysithea*. Separated from her closest friend, Noboru Terao, their bond persists through text messages, though interstellar distances soon warp their communication. Initially exchanging words within minutes, the delays expand into years as Mikako ventures toward the Sirius Solar System. Time dilation freezes her at sixteen, while Noboru ages into adulthood, their lives diverging across the stars.

Amid perilous battles, Mikako composes messages haunted by isolation, her longing intensifying with each mission. During a pivotal clash on Sirius’ fourth planet, Agartha, she transmits a final confession of love—a message destined to drift through space for nearly a decade. Her duty exacts a heavy toll, manifesting in hallucinations where she desperately reaches out to Noboru, torn between her role as a soldier and her yearning to reconnect.

Expanded narratives reveal Noboru’s choice to join a UN rescue operation, striving to bridge the chasm between them. Despite her Tracer’s crippling damage in battle, leaving her stranded in the void, Mikako clings to hope upon learning of his efforts. Flashbacks to their mundane past—shared moments at convenience stores—underscore the stark contrast to her militarized present. The manga’s conclusion hints at unresolved reunion, her resolve unshaken even as cosmic forces conspire to keep them apart. Through every adaptation, her arc remains anchored in the tension between sacrifice and desire, a testament to resilience against insurmountable odds.