Teenage math whiz Kenji Koiso 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 their matriarch. His job: pretend to be Natsuki's fiancé. Meanwhile, his attempt to solve a mathematical equation causes a parallel world's collision with Earth.
The story begins when the protagonist, a 14-year-old girl named Ai Ohto, hears a mysterious voice while walking around town late at night. It gives her an egg, and beckons to her: "If you wish to change the future, you need only choose now. Believe in yourself, and smash the egg."