Third-year middle school student Aoi is the daughter of a wealthy company president, living a life that others might envy, but she harbors a devastating secret. She has a weak heart and has been given approximately five years left to live, a prognosis she has decided to reveal to no one as she approaches graduation. Overwhelmed by despair after receiving this diagnosis, she runs away into the night city and encounters Saki, a boy her own age who is a lone, aloof band musician.

Believing their meeting to be a fleeting, one-time moment, Aoi confides her secret to him. However, she is surprised to later reunite with him at school, where she learns he is a classmate with a reserved and seemingly unfriendly exterior. Despite his standoffish demeanor, Saki reveals a hidden gentle side, telling her, "You can show your weakness only to me". As Aoi finds herself drawn to him, a poignant truth is revealed just as she is about to confess her feelings. The narrative follows their bittersweet and devoted connection as Aoi attempts to experience a lifetime's worth of love within her limited remaining time.