High school student Kenji Koiso agrees to accompany his friend Natsuki to her grandmother’s birthday celebration. At the same time, he accidentally cracks the security code of OZ, a vast virtual world that controls much of the planet’s infrastructure. Kenji’s mistake triggers a massive system failure, putting countless lives at risk. Now the young mathematical prodigy must find a way to set things right. Joining forces with Natsuki’s large family, he embarks on a race to save the world.