A heart can only beat so many times in a lifetime. Kokuhaku disease, a rare condition that weakens the heart, leaves the victim with less heartbeats and a shorter lifespan than normal. With the right device, the amount of heartbeats remaining can be measured.

When Hato Iijima accidentally walks in on Hatsu Takagi changing her clothes, he sees this device attached to her chest. Hatsu has resigned herself to an uneventful and isolated life to avoid elevating her heart rate. However, she expresses her true desire—to freely and wholeheartedly experience every second she has left. Recognizing her wish, Hato swears to help her enjoy life to the fullest and make her heart beat like crazy.