Takuma and Mayu's story began when they were only 8 years old… Mayu (Mao Inoue) is the young daughter of a heart doctor and Takuma (Masaki Okada) is a boy being treated by her father. The two children meet, become firm friends, and become each other’s first love. Takuma and Mayu overhears Takuma's prognosis, that he will not live past the age of 20. The two distraught children make a promise for an uncertain future. 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 her. He cannot stand to see her cry or hurt her more than he already has. What will become of their love while the clock keeps ticking away at Takuma’s life?