The background of the story explains that humans landed on the moon long ago intending to create a new home for themselves called the Kingdom of Sphere. However, the relations between Earth and the Kingdom have worsened things and a war called the Oedipus war started. Once the war was over, it was decided that the city of Mitsurugasaki in Shizuoka Prefecture of Japan, where the story takes place, under the rule of the United Nations, be developed as the center of diplomacy with the Lunar Kingdom, which, currently, has closed its doors to the world. This central lunar junction port city, which has the only spaceport within the United Nations with ships going to and coming from the Lunar Embassy, is, as its name suggests, the gateway which connects the Earth and the moon. Although tensions have temporarily eased now, relations between the two worlds is still in a situation when the story begins where caution must still be taken.
