Naoki Kazumi is a boy whose parents died in an accident five years ago, and his memory went along with it. Since he had nowhere else to go, he lives with his cousin Matsuri Shibugaki and her parents. Every morning before school, he is awakened by his childhood friend, Honami Fujieda, who keeps telling him that he is not a child anymore, but still comes to wake him up. These usual, ordinary days seem to stretch on forever, until one day up on the school's rooftop, while Naoki is taking a peaceful nap, out of nowhere, a girl falls out of the sky. This girl, called Mikoto Amagasaki, comes from 100 years in the future, and she went back in time with the intention of finding her brother. When Mikoto sees Naoki, she mistakes him for her brother.
