A high school girl, Sayaka Kudō was the bottom of the class. After a year, she improved her deviation value from 30 to 70, then passed Keio University that is considered one of the most difficult to enter in Japan.

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.