This story revolves around Takuma, a boy who is told he will die before he is 20, and Mayu, a girl who is in love with him.
The story begins when they are little kids. Takuma Kakinouchi and Mayu Taneda were friends since they were 8-years-old. Takuma has a heart disease and he is treated by a cardiologist who happens to be Mayu's father. Their friendship grows and Takuma becomes Mayu's first love.
Time passes, and the girl becomes a young woman, and the boy, a young man. Mayu continues to support and love Takuma. On the other hand, Takuma, who knows his days are numbered, pushes away his feelings for Mayu and distances himself from Mayu, knowing that he cannot keep his childhood promise because he cannot stand to see her cry or hurt her more than he already has.
Then Mayu meets Kou Suzuya, who has feelings for her, and Takuma meets Teru Uehara, one female patient who suffers a similar illness,who has a crush on him and their feelings for each other begin to waver.