Naoki Kuzumi lost his parents in an accident five years ago, and is living with his aunt, uncle, and cousin Matsuri. He always thought his life was ordinary, aside from the fact that he can't remember what happened in his youth. One sunny day as he is taking a nap on a bench, a redheaded girl -- Mikoto -- literally falls on him from the sky. For some reason, she thinks he's her younger brother.