The virtual world of OZ has taken Japan by storm. Constructing an avatar in this real-world simulation grants users access to a wide range of features—from playing games to processing legal documents. Due to its overall convenience, OZ's popularity has skyrocketed since its inception and has been deemed the world's greatest communication tool.

Introverted OZ moderator Kenji Koiso had decided to devote his summer vacation to overseeing the system's maintenance. However, he changes his mind after being invited to accompany fellow student Natsuki Shinohara to her family's residence, where the Jinnouchi clan is gathering for the impending 90th birthday celebration of her great-grandmother.

Upon their arrival, Natsuki introduces Kenji as her fiancé, which comes as a shock to both himself and her great-grandmother. While struggling to keep up with the actions of his crush, he receives an anonymous text. Enticed by the complexity of its encryption, he decodes the message and sends a response—unknowingly bringing about a great catastrophe for OZ that will have dire consequences.

[Written by MAL Rewrite]