Thanks to a remark by my high school classmate Yamauchi Sakura (Minami Hamabe), I (Shun Oguri) eventually became a teacher at my alma mater. While chatting with my student Kuriyama, I inadvertently recalled the months I spent with Sakura. Back then, I (Takumi Kitamura) accidentally discovered Sakura's secret. She suffered from a serious pancreatic disease, and I don't know when she started writing a diary to fight the disease. The disease did not knock down this lively and cheerful girl, but instead she showed an increasingly brilliant smile. In contrast, I lived more in my own world and rarely interacted with others. With the help of the diary, our life trajectories briefly intersected. I spent every limited day with her until that happiness and freedom came to an abrupt end... This film is adapted from the novel of the same name by Yoru Sumino.