28-year-old Kyung Hwi is a resident at a university hospital. He was bullied in high school. He made a suicide attempt, but Han Na Bi, a mysterious girl that transferred to his school early that semester, prevented him from dying. However, after telling him to live, she had suddenly committed suicide. Kyung Hwi couldn’t forget about her for the past decade and one day wakes up to find himself in his old self 10 years ago. It’s a week before Na Bi committed suicide. He is the 18-year-old loser he used to be, but he can’t miss this opportunity. He has to find out what happened to Na Bi, why she killed herself, and stop her from doing so. Kyung Hwi goes through his life as an 18-year-old with the experience of a 28-year-old, changing his gloomy past as he goes. Will Kyung Hwi, who went back to the time he was an outcast, be able to save his first love and have a happy ending? (Source: JTBC)