The world of OZ—a virtual space the connects the entire world—has become extremely popular. Accessible to almost everyone, the "citizens" of OZ can customize their own avatar and engage in a multitude of leisurely activities that range from shopping to socializing. Thanks to many more convenient features, the majority of the world is heavily reliant on OZ—the fire brigade uses it to respond to emergencies, doctors can monitor a patient's vitals whenever they please, and it is even trusted with safely returning the asteroid explorer, Arawashi.

Kenji Koiso is a math genius and a part-time OZ moderator. He is invited by Natsuki Shinohara, his school's idol, on a summer trip. However, he did not expect to have to act as her fiancé for her grandmother's 90th birthday party. Shortly after his arrival at Natsuki's family estate, Kenji receives a strange coded message on his phone from a sender who challenges him to solve it. Using his math expertise, Kenji solves the 2,056 character code in just one night—but little does he know about the chaos that will be unleashed...

[Written by MAL Rewrite]