OZ, a virtual world linked to the internet, has gained immense global popularity as a venue for users to participate in diverse activities, such as sports and shopping, via self-created and customized avatars. OZ also features nearly impenetrable security thanks to robust encryption, ensuring that personal data transmitted through its networks remains safe to protect its users. Due to its convenient applications, most of society has become heavily reliant on this simulated reality, even entrusting the system with the retrieval of the unmanned asteroid explorer, Arawashi. Kenji Koiso is a 17-year-old math prodigy and part-time OZ moderator who is invited by his crush, Natsuki Shinohara, on a summer trip. However, he is unaware that this adventure requires him to pose as her fiancé. Shortly after arriving at Natsuki's family estate, which is preparing for her great-grandmother's 90th birthday, he receives a strange, coded message on his cell phone from an unknown sender challenging him to solve it. Kenji manages to crack the code, but little does he know that his mathematical expertise has just placed Earth in grave danger.