Kyuta, a boy living in Shibuya, and Kumatetsu, a lonesome beast from Jutengai, an imaginary world. One day, Kyuta forays into the imaginary world and, as he's looking for his way back, meets Kumatetsu who becomes his spirit guide. That encounter leads them to many adventures.
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.
After her werewolf lover unexpectedly dies in an accident, a woman must find a way to raise the son and daughter that she had with him. However, their inheritance of their father's traits prove to be a challenge for her.
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."