A student tries to fix a problem he accidentally caused in OZ, a digital world, while pretending to be the fiancé of his friend at her grandmother's 90th birthday.

Kenji Koiso, an eleventh grade math genius, agrees to take a summer job at the Nagano hometown of his crush, Natsuki. When he arrives, he finds that her family have reunited to celebrate the 90th birthday of the family matriarch.

His job is to pretend to be Natsuki's fiancé. Meanwhile, his attempt to solve a mathematical equation causes a parallel world's collision with earth.