The virtual realm of OZ has swept through Japan. Building an avatar within this simulation of the real world provides users with access to a broad spectrum of functions, ranging from gaming to the processing of legal paperwork. Because of its general convenience, OZ's popularity has surged since its launch, earning it the title of the world's premier communication tool. Kenji Koiso, an introverted moderator for OZ, had planned to spend his summer break managing the system's upkeep. Yet, he alters his plans after receiving an invitation from his classmate Natsuki Shinohara to visit her family home, where the Jinnouchi clan is assembling for the upcoming 90th birthday party of her great-grandmother. Once they arrive, Natsuki presents Kenji as her fiancé, a revelation that startles both him and her great-grandmother. As he tries to keep pace with the behavior of the girl he likes, he gets a text from an unknown sender. Drawn in by the message's complex encryption, he cracks the code and replies, inadvertently triggering a massive disaster for OZ with severe repercussions.