Sung Hae-sung (Yeo Jin-goo) is a senior high school student. On Hae-sung’s birthday, his girlfriend Jung Jung-won (Jung Chae-yeon) prepares a surprise party for him at his house. To keep Hae-sung away from the house while they prepare, Jung-won asks him to go to their school and retrieve her wallet. There, he walks into a shocking scene. While trying to get help, he is hit by a car and dies. 12 years later, Hae-sung wakes up at the roof of his school building, and he meets Jung-won (Lee Yeon-hee) again. Although Jung-won and Hae-sung were born in the same year, the two childhood friends end up being 12 years apart because they have aged at different places (worlds) for some strange reason. Now Jung-won is a 31-year-old woman, but Hae-sung is still a 19-year-old boy. With the assistance of his friends and Jung-won, Hae-sung sets out to make things right for his family and solve the mysteries surrounding his death and resurrection.