Everyone is reminiscing about their lost youth, hoping to return to the age of 18, make up for the regrets of the past, and complete the unfinished business. But for the now 30-year-old doctor Xu Yunzai (played by Gong Yoo), the soul swap caused by the car accident made him a 30-year-old uncle on the surface, but an 18-year-old boy in his heart, which should not be so exciting. Because of this swap, Xu Yunzai actually became a high school student Jiang Jingjun (played by Shin Wonho). Looking at the unfamiliar face in the mirror, Jingjun, who became Yunzai, was eager to find a way to return to his true body at first, but he couldn't find it for a while. So Jingjun had to start a new life uncomfortably, and the first thing he had to face was Yunzai's fiancée, Ji Dalan (played by Lee Minjung), who was also his teacher. Facing the drastic change in her fiancé's temperament, Duolan was very uncomfortable. And this is just the beginning of the story...