The girl that fell out of the blue. And the protagonist's life, which began to take a turn little by little into the realm of the extraordinary starting from that very day. 「Nao-kun, if you don't get up soon, you're going to be late.」 The new school term started today. The protagonist's childhood friend, Honami, has kept telling him that he's not a child anymore, and yet continues to come to wake him up. They proceed as they always have to 『Hasumidai Academy』, which stands atop a hill ... but this time around, as second-year students. 「Naoki, I'm going on ahead of you!」 And the protagonist's cousin, Matsuri, zips past the two of them. After having lost both parents in a terrible accident 5 years ago, the protagonist Naoki Kuzumi was taken in by Matsuri's parents. And now, Matsuri herself -- who had been so opposed to Naoki's moving in back then -- has become a freshman at Hasumidai Academy. These ordinary, ordinary days, stretching on forever. ... but then, one day, up on the roof of school ...