Growing up as the best of friends, Sang-doo (Bi) and Chae Eun-hwan (Gong Hyo Jin) make lots of lasting memories during their shortened high school years. When Eun-hwan's family goes into bankruptcy, debtors rush into their house to grab all of her family's belongings away. When somebody takes her father's old record player that she really loves, Sang-doo goes after the guy. In a physical fight, the man falls off the bridge and enters a coma. Eun-hwan, not knowing this, runs away from the town. Sang-doo goes to jail, returning to find out that his family and Eun-hwan have left him. Years later, he becomes a gigolo (with his uncle) in order to raise the hospital bills for his sick daughter (Se-ra (Hong Soo Hyun) claims that Bori is her daughter). Eventually, he meets his long-lost first-love, Eun-hwan as a high school math teacher. In an effort to never lose her again, he returns as a high school student (to finish his last years of high school) and gets placed in Eun-hwan's class. In numerous funny yet heart-touching moments, Sang-doo vows never to become a gigolo again. Yet, his past deeds have caught up with him, and he is soon sought after by the police. Eun-hwan's boyfriend, Min-suk (Lee Dong Gun), also does not want to let go of Eun-hwan. First love is something beautiful yet painful to these two lovers, who have reunited after ten years to find out that everything is not the same anymore.