Dae Su and Mi Ra became parents when they were only teenagers, and at times, the hardworking couple still come across as immature. They have, however, the world‘s most mature child. Born with a rare genetic disorder, Ah Reum ages far faster than everyone else. At age 17, he has the frail body of an 80-year-old. As his birthday nears, Ah Reum, sensing that he may not have much time left, prepares a gift for the parents who have raised him with such love and care.