Nagamine Mikako and Terao Noboru are close friends who dream of entering the same high school together. In the summer before the entrance exams, Mikako tells Noboru that she has been selected for the crew of the UN Space Forces and will be traveling into space. Their final destination is a planet on the very edge of the Solar System. Separated by sky and earth, they continue to stay in touch, but the vast distance of light years – never before experienced by humans—creates a bottomless chasm between their time and their hearts.