Immature and clumsy Dae-soo and beautiful but foul-mouthed Mi-ra were both seventeen-year-old teenagers when Mi-ra became pregnant. As a result of familial rejection, Dae-soo decides to run away from home and care for his family on his own. Their son Ah-reum was born, but he was diagnosed with the extremely rare genetic disorder progeria, which makes its recipient senescence prematurely and rapidly. Years later, Ah-reum is sixteen, but his body is that of an eighty-year-old man. The family faces the fact that he may not live to see his 18th birthday. Sensing that he doesn't have much time left, Ah-reum writes a story about how his young parents fell in love and how he came to be, hoping to give it as a gift to them on his seventeenth birthday. At the same time, Dae-soo and Mi-ra struggle to raise money for Ah-reum's hospital expenses and accept the reality that their son will not be with them much longer. To cover costs, Dae-soo takes up work as a taxi-driver and Mi-ra works at a laundry. Dae-soo struggles to balance work and spending limited time with his son. Hearing the story of Ah-Reum, a television crew searches out the family to make a documentary program about Ah-Reum and his condition. After the documentary is screened, Ah-Reum begins to receive emails from another child who is sick, a young girl whose messages comfort him and provide hope as he tries to recount the love story of his parents as his final gift to them.