Narrated by an unnamed character, the novel unfolds when he stumbles upon a diary in a hospital, penned by Sakura Yamauchi, his classmate. Sakura discloses her terminal pancreatic illness with only a fleeting time ahead. Entrusted with this confidential knowledge, the protagonist commits to preserving her secret. Despite their contrasting traits, the two forge a bond as the protagonist chooses to stand by Sakura during her remaining days.