context: Hyun-soo (Kwon Sang-woo), transfers to another school, which is notorious for being one of the worst and most brutal high schools in the country. He is sent to a bottom-rank class and experiences violence in classroom by a group of delinquents in his class led by Jong-hoon (Lee Jong-hyuk), who is a stereotypical Korean bully in school. The teachers are also depicted as authoritarian and brutal to the students, using discipline as an excuse for brutality, as did most Korean teachers up to the 70s. One day on the bus, Hyun-soo sets eyes on and falls in love with Eun-joo (Han Ga-in), and one day when a group of schoolboys start teasing her on the bus, Hyun-soo takes the opportunity to gain her respect, even though it meant starting a riot. After a chase through the neighborhood and being caught in their hiding place (a rat prompted Eun-joo to scream), one of Hyun-soo's friends Kim Woo-sik (Lee Jung-jin), who happens to be a son of a popular actress, steps in and knocks them out. A relationship triangle was formed between Hyun-soo, Woo-sik, and Eun-joo. One day Hyun-soo while riding his bike noticed Woo-sik and Eun-joo arguing. Taking into account that Hyun-soo was at a disadvantage of winning her heart due to his only achievement being sharing an umbrella with her on a rainy day while Woo-sik did more, he meets her at a rooftop that night and bonds with a romantic song playing on the radio presumably bought for the occasion, much to Woo-sik's jealousy when he saw them unnoticed when he reached the rooftop. One day Hyun-soo and Woo-sik got in a fight over Eun-joo and their friendship was put to an end, which made Woo-sik the tough person he is even worse, going to the extent of humiliating and beating Ham Jye-bok who is called by his nickname Hamburger (Park Hyo-jun), who sells pornography to other students. He leaves the school and presumably drops out after losing an intense fight with Jong-hoon on the rooftops, never to be seen again. Hyun-soo is now facing greater problems of his own when he concludes Eun-joo left him for Woo-sik again and he has been receiving poor grades prompting his father, a black belt in Tae Kwon Do and an owner of a Tae Kwon Do gym, to berate him and resort to extreme violence to 'discipline' him and labeled him a 'surplus man', a derogatory term used to describe a person without a future, and told him to leave his house which Hyun-soo out of anger did and was almost driven to suicide. No longer able to put up with the conduct of Jong-hoon and his boys as well as the extreme brutality and violence being put up by the teachers, Hyun-soo starts to train himself in Jeet Kune Do, inspired by his childhood hero Bruce Lee. Hyun-soo challenges Jong-hoon to a fight one day when he is no longer able to put up with his irrational behaviour. On the same rooftop where Woo-sik lost a fight, Jong-hoon and his gang mates are grievously injured by Hyun-soo's nunchaku and expertise in Jeet Kune Do. After the intense fight, although he won, Hyun-soo still didn't feel any victory. Authorities including a general who is called 'puppet soldier' by students were alerted and they started yelling at him even though it was Jong-hoon who was antagonizing others. At this point, badly influenced by the brutality from his psychopath father and still being accused rather than honored after all this time and effort he invested to master Jeet Kune Do for the sake of bringing justice to the school and proving himself wrong of his status as a surplus man, Hyun-soo totally lost it and smashed some nearby windows and hurled his nunchucks at them and cursed Korea's entire education system for being so corrupt and encouraging brutality no matter how immoral it is, leading to expulsion. Later on, Hyun-soo and his father, who has heard everything of what happened as well as what his son has been practicing, pays a visit to a hospital to meet the now hospitalized Jong-hoon (along with his gang mates) and his furious mother. Hyun-soo's father's begs her to forgive him calmly and sincerely without even the slightest bit of anger and sees the error of his ways by admitting his method of discipline was wrong as they only made his bad enough son even worse; realizing the result of bad parenting and child abuse. On their way home, Hyun-soo apologizes to his father, and in response he questions him if Bruce Lee ever went to college and silently walked home without a word. Sometime later, Hyun-soo is now attending a GED institute with teachers who don't utilize brutality upon students, and met Hamburger in the hallways after class, who was recently expelled right before graduation, in the hallways and discussed about their past. Hamburger explains that their friends are doing fine but has not heard from Woo-sik at all. He also says he spotted Eun-joo at the institute where she's restudying for college. One night on the bus, Hyun-soo meets Eun-joo again, but they say their goodbyes. The film ends with Hamburger and Hyun-soo outside of a movie theater showing Jackie Chan's 1978 movie Drunken Master, rough-housing as they argue about Jackie Chan and Bruce Lee.
question: What does Hamburger sell?
answer: pornography