In the near future, where eyes have been mechanized to perceive augmented reality (AR) indistinguishably from reality, there exists a city known as 'District Zero'. In this fully AR-compatible city, residents not only enjoy a prosperous life and stimulating entertainment but also benefit from the management of their visual experiences by the AR network called 'ADONIS' making it touted as the safest city in the world.

Within District Zero, the protagonist, Kurauki Shion, works as a first-class investigator for the ADONIS corporation, safeguarding the city from potential threats. However, after becoming entangled in a conspiracy surrounding the 'Swampman Incident' he was assigned to investigate, Shion is demoted to a sinecure position at the OCTA branch.

Amidst the despair, Shion encounters a peculiar situation where his childhood sweetheart Moka, presumed to have died in childhood, appears before him in the form of a child on the AR platform. She exists as an electronic ghost with no physical presence in the real world.