31-year-old Lin Yang is great at her job but still lives alone. Her life is currently stable but boring. 24-year-old Lu Zheng'an, her younger childhood neighbor, suddenly returns from studying abroad and enters Lin Yang's world. Their 24-hour interaction completely changes Lin Yang's life. With Lu Zheng'an's care, she "mentally ages backward", and they begin marching towards happiness together. From a younger brother to a boyfriend, from growing up to growing old together, he learns to be responsible and dependable for her, while she keeps a younger heart and learns to rely on him. Even though life is hard, I'm thankful to have you as my sweetie.