Naoki Kuzumi, a high school junior, lost his parents in an accident five years ago and now lives with his aunt, uncle, and cousin Matsuri. Aside from the fact that he can't recall what occurred in his youth, he always assumed his life was typical. One bright day, when he is napping on a bench, a redheaded girl named Mikoto practically falls from the sky on him. She believes he is her younger brother for some reason. Naoki's time period serves as a temporary refuge for individuals in the future, when many are afflicted with an incurable sickness. Mikoto had gone back in time after hearing that her younger brother Yuusuke had been brought to Naoki's time.