In the year 2315, humanity is defeated in a war against Machine Intelligence, sentient machines born from an accident, and is driven from the surface of the Earth. Forced to hide in massive underground shelters, human society gradually degrades over the centuries. By the year 2824, nations have collapsed, and vigilante groups known as Security Forces control the remaining people and facilities. Humanity has stopped counting the calendar and resigns itself to slow extinction.

The story centers on Tenshu Mikoto, a blind girl who once fought as a soldier for the Security Forces. Now an outcast or Outer who does not belong to any community, she earns a living as an engineer in the discarded sections of the shelters. She works alongside her companion Noriko, salvaging and repairing machines and batteries. Mikoto also pilots and manipulates a Security Forces Organoid, the 13th Generation Cradle F/V-10 Honet-C, a type of mecha.

The narrative focuses on the deep bond between the android Noriko and her blind friend, Mikoto. Their central conflict arises from their shared dream: to one day stand together under the blue sky, a concept they have only heard of in legends from the past. To achieve this, Noriko must fight against the system of war machines that blocks their path to the surface, willing to sacrifice everything to fulfill her friend's wish. The storyline uses the mecha and the oppressive underground environment to explore themes of unbreakable bonds, the will to sacrifice for another, and the desperate hope to reclaim a lost world. The anime is a sequel to the earlier work Gekijōban Vapor Trail and is a standalone story within the larger Vapor Trail series, serving as a passion project by the independent creator AsH.