A world where mysterious objects known as "artifacts"—capable of triggering phenomena that defy human understanding—have appeared. Humanity lives divided between two realms: the Upper World, where a colossal city has been built, and the Lower World, a vast and ruined wasteland. While carrying out a solo mission in the Lower World, Tezuka, a member of a special task force that collects these artifacts, searches for clues to find his missing sister...