Young Haru rescues a cat from being run over, but soon learns it's no ordinary feline; it happens to be the Prince of the Cats.
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.
In an abandoned Tokyo overrun by bubbles and gravitational abnormalities, one gifted young man has a fateful meeting with a mysterious girl.
