Everyone is reminiscing about their lost youth, hoping to return to the age of eighteen, make up for the regrets of the past, and complete unfinished things. But for the 30-year-old doctor Seo Yoon-jae (played by Kong Yoo), the soul exchange caused by a car accident turned him into a 30-year-old uncle on the outside, but an 18-year-old boy on the inside...