Middle school students Mikako Nagamine and Noboru Terao are close friends and members of the same club activities. But in the summer of 2046, Mikako tells Noboru that she has been chosen as a Select Member of the United Nations Space Army. Mikako leaves Earth in the winter of 2047, while Noboru goes on to high school in Japan. Separated by space, the two try to stay in touch by mobile mail. However, as Mikako goes farther out in the solar system on board the Lysithea spaceship, it takes increasingly longer for each to receive the other's mail. Noboru stays on Earth, frustrated by his situation and resigned to the fact that he can only wait for Mikako's emails. As time goes by, the Lysithea fleet approaches a time warp, a situation that make both Mikako and Noboru acutely aware of the time gap separating them.