Sang Doo! Let's Go to School.

Orphaned at young age, abandoned by his uncle and later by his adoptive parents, Sang-doo’s life has been a succession of difficulties. At 27, he makes a living by seducing older women in order to support his daughter’s hospital fee. However, the love he has for his first love, Chae Eun-hwan, keeps him happy and optimistic. When he runs into Eun-hwan again, he decides to give another chance at his love and goes back to high school to be Eun-hwan’s student.