电影根据佐野徹夜的小说改编，在姐姐去世后，卓也一直过着浑浑噩噩的生活，直到他遇见了患有不治之症「发光病」的少女·真瑞，为了帮助少女完成心愿，卓也与少女许下约定。Takuya Okada is a high school student. He visits his classmate Mamizu Watarase in the hospital. Mamizu suffers from the mysterious ailment "Luminescence disease." People who have the disease show a faint sparkle under the moonlight and the sparkle gets brighter as their death gets closer. Mamizu is not allowed out of the hospital and she will not live long enough to become an adult. Takuya offers to help her carry out her wishes before she passes away. He also tells her how he feels about her, but Mamizu's death is approaching.