The story begins on December 31, 9999, when a young amnesiac girl by the name of Mion wakes at the bottom of a dark ruins within a desolate world. Following a desire to let Mion escape the rubble and see the outside world, the firefly Hotaru leads her on a journey upwards through the ruins. On her journey outwards, she finds unmanned machines, distorted vegetation, and bodies of dead children.

The story begins on December 31, 9999, when a young amnesiac girl by the name of Mion wakes at the bottom of a dark ruins within a desolate world. Following a desire to let Mion escape the rubble and see the outside world, the firefly Hotaru leads her on a journey upwards through the ruins. On her journey outwards, she finds unmanned machines, distorted vegetation, and bodies of dead children.