Lim Sejun, who likes to eat, is fat and has a large physique. He has high self-esteem and a firm personality that doesn't even care about being teased for being fat. He has only one friend, 'Seo Hajin'. She has always been learning Jiujitsu to protect Sejun since childhood. One day, Sejun has trouble with a delinquent in the same class, and Hajin sees it and destroys that delinquent. But one night, Hajin is found severely beaten and suffers from a badly injured knee and more serious psychological trauma. Sejun tries to tell the police, but when the investigation doesn't make much progress, he can't control his anger and goes to find the culprit himself, but he's defeated by the delinquent who learned Jiujitsu and finds nothing. Then he learns Jiujitsu from a man named 'Jason' who saved him. Three months later, Sejun, who learned Jiujitsu and lost weight, finds out that a veiled group is involved, and wants to destroy it. A school action drama depicting individual revenge.