Five years ago, Naoki Kuzumi lost his parents in an accident and has been living with his aunt, uncle, and cousin Matsuri while studying as a high school junior. Although he struggles to remember his childhood, he feels his life is relatively ordinary. However, his life takes an unexpected turn when a red-haired girl named Mikoto falls from the sky and lands on him while he naps on a bench. To his surprise, she mistakes him for her younger brother. Naoki soon learns that his time period serves as a haven for people in the future who have incurable illnesses. Mikoto traveled back in time to find her younger brother Yuusuke, who was transported to Naoki's time.