Twenty Years Old is a romantic drama that follows the story of Kim Hye-rim, a 20-year-old college student who has never experienced a real relationship. Her life takes an unexpected turn when she reunites with her middle school classmate, Lee Gi-kwang, who has since become a famous member of the idol group BEAST. As the two navigate the complexities of their rekindled connection, they must manage the pressures of Gi-kwang's celebrity status and the public eye. The series explores the themes of first love, the challenges of fame, and the transition into adulthood, highlighting the sweet and often difficult moments of a secret romance blossoming in the spotlight.