OZ, a popular virtual world connected to the internet, offers a diverse range of activities for users to engage in through their personalized avatars, such as sports and shopping. With its robust encryption, OZ ensures the safety of all personal information shared over its network. Consequently, society has become reliant on this simulated reality, even entrusting it with the return of Arawashi, an unmanned asteroid explorer. Kenji Koiso, a brilliant math whiz and part-time OZ moderator, accepts an invitation from his crush Natsuki Shinohara to join her on a summer trip. Unbeknownst to him, their adventure involves him posing as her fiancé. Shortly after arriving at Natsuki's family estate, where preparations for her great-grandmother's 90th birthday are underway, he receives a cryptic message from an unknown sender challenging him to solve its code. Kenji manages to crack the message, but he remains unaware that this act puts Earth in perilous danger.