Cha Sang-doo (Rain) and Chae Eun-hwan (Gong Hyo-jin) were best friends growing up, who secretly have crushes on each other. One day, Eun-hwan's family goes bankrupt and debtors rush to collect their belongings. Eun-hwan tries to reason with a debt collector who is taking her father's old record player, but he takes it anyway. Sang-doo, on his way to walk to school with Eun-hwan, sees the whole thing. He tries to purchase it from the man, but is refused. In a struggle to retrieve the record player, Sang-doo ends up throwing a man off a bridge and putting him in a coma. Sang-doo is sent to prison for assault. After being released from prison, he finds that his parents have disowned him and moved to America and Eun-hwan has run off with her family.
Years pass, and Sang-doo now has a daughter, Cha Bo-ri (Song Min-joo), from a one night stand with Han Se-ra (Hong Soo-hyun). She is sick and diagnosed with leukemia. In order to pay off her hospital bills, Sang-doo becomes a gigolo. His uncle (Lee Young-ha) was a former gigolo and gives him advice on the trade.
One day, he comes across his long-lost first love, Eun-hwan, who is now a high school math teacher in his city. In an effort to regain her love, he returns to high school, first as a security guard and then as a student in her class. Unfortunately, she is engaged to his daughter's doctor, Kang Min-suk (Lee Dong-gun). Min-suk and Sang-doo start off as rivals, vying for Eun-hwan's heart, but later end up as good friends. As everything seems to be going smoothly for Sang-doo and Eun-hwan, circumstances prevent them from staying together.
Sang-doo is the protagonist of the drama. As a boy, he was charismatic and always popular. One day, he tried to retrieve his first love's cherished possession, but in the process kills a man. His life was turned upside down - he was sent to prison, his parents disowned him, and his first love disappeared. He ends up having a child with Han Se-ra. His daughter Bo-ri has leukemia and to pay her medical bills, he secretly becomes a gigolo with the help of his uncle.
Eun-hwan has been in love with her childhood best friend since high school, but was separated from him due to her family's financial troubles. As a sign of her devotion to him, she takes care of his dog until the day they reunite.
Min-suk is Eun-hwan's fiancé and also happens to be Cha Bo-ri's attending physician. Initially, he dislikes Sang-doo and always finds ways to challenge him. Eventually, he gets to know Sang-doo's true character and becomes one of his best friends.
Se-ra is a wannabe model. She was abandoned by her birth mother when she was young and has lived on her own ever since. She met Sang-doo at a restaurant and fell in love with him. After giving birth to Bo-ri, she gave her up for adoption because she was unable to pay for her medical bills. Sang-doo retrieves Bo-ri and raises her. Se-ra uses this to stay close to Sang-doo.
