This movie is about Kim Ji-hoon and Choi Su-wan both 21 years old. Su-wan is in her second year at university while Ji-hoon is still in the process of repeating his senior year of high school.
Su-wan's mother works in their family business, selling fried chicken. To help out, Su-wan earns money by tutoring other children, but often gets fired after hitting the students. Her mother has a rich friend with a son in need of a tutor. The friend, Kim Ji-hoon's mother, wants a tutor that will make her son study so that he can pass his high school graduation exams. The salary offered is enough to pay for an entire semester of Su-wan's college education.
Ji-hoon is always able to attract girls and draw attention through his love of fighting. He has recently transferred to Surim High School, and manages to offend Lee Jong-soo
"king" and best fighter there. Jong-soo's girlfriend, Yang Ho-kyung (Kim Ji-woo) also sets her eye on him. He continues to brawl at Surim, including beating up a member of the high school gang for stealing from his younger brother Se-hoon. Apart from studying, Ji-hoon is actually fairly well rounded. As a child, he was sent to America to study. Because of his parents' decision, the relationship between them and Ji-hoon is strained. Ji-hoon's father, Kim Bong-man threatens to send him back to the US if he continues to get into trouble with his fighting and bad grades. To avoid being sent back to America, Ji-hoon needs to get at least a 50% on the upcoming mid-term exams. The only problem is that his usual average mark is much lower than that.
Su-wan is persistent in tutoring Ji-hoon despite his rudeness to her, and eventually their relationship improves. At the end of the term, Ji-hoon receives a barely passing grade (50.1), but passing nonetheless. According to the terms of a bet they made, Su-wan must dance in front of a crowd during a college festival. While embarrassed and shy at first, she eventually really gets into dancing and the audience cheers her on. Ji-hoon realizes his growing feelings for Su-wan when he gets jealous and angry at seeing all the guys watching her running up to the stage.
Then Su-wan's boyfriend comes back home. While spying on the couple, Ji-hoon learns that Su-wan's boyfriend is breaking up with her. The next day, Ji-hoon shows up at his house and beats him up for hurting her. However, he finds out that the reason he was leaving Su-wan was because he was entering the priesthood. After the ordination, Ji-hoon lets Su-wan vent her feelings by hitting him for a while. Once she settles down, he decides to take her sky sailing. While in the air, Ji-hoon tells Su-wan that he likes her. Su-wan does not know how to respond and pretends that she couldn't hear him over the earpiece. Later on, they go to an amusement park. Just as Ji-hoon is about to give Su-wan a present, Su-wan runs into a female college friend, who's accompanied by her boyfriend. Su-wan had previously confided in the friend about her difficulties in tutoring Ji-hoon. Su-wan's friend initially thinks that Ji-hoon is her boyfriend, but Su-wan denies it, saying she would never go out with a high school student. This angers Ji-hoon and he throws her present into the fountain and leaves.Eventually, all of Ji-hoon's fighting catches up with him. Ho-kyung and her clique, Jong-soo and his high school gang, and actual gang members confront him while holding Su-wan hostage. Ji-hoon and Su-wan manage to run away, but they eventually get cornered and Ji-hoon has to fight everyone on the beach. Once all of the other gang members are down, Ji-hoon faces the mafia's best fighter. Ji-hoon is at the man's mercy, but Su-wan saves Ji-hoon by kicking the man in the groin. Then Su-wan and Ji-hoon drive away on her chicken delivery scooter, with her wearing the necklace, Ji-hoon's earlier gift. At the end, Ji-hoon and Su-wan are now a couple, and she is tutoring his younger brother Se-hoon.
