Summer War (English subtitle)

OZ, a virtual realm linked to the internet, is celebrated worldwide for its diverse array of activities, including sports and shopping, all of which may be pursued via customizable avatars. Additionally, OZ boasts near-invincible security due to potent encryption, ensuring the safekeeping of transmitted personal data to protect users who depend on it. Such reliance on the simulated universe has led society to entrust the unmanned asteroid explorer, Arawashi, to the system.
Seventeen-year-old math whiz and part-time OZ moderator, Kenji Koiso, is invited on a summer outing by his crush and co-worker, Natsuki Shinohara, but is later surprised to discover that he must pose as her betrothed. Upon reaching Natsuki's family estate, where the family is preparing to commemorate the matriarch's 90th birthday, Kenji receives a cryptic message on his mobile phone from an unknown sender that offers him a challenge. Though he cracks the code with ease, Kenji is unaware that his grasp of mathematics has put Earth in grave peril.