Na Du-su (Lee Seung-hyeon), who sports the nickname of "Yalkae" (meaning "cheeky, cocky person"), has been a high school sophomore for two years. His partner-in-mischief Yong-ho (Jin Woo-young), whose father runs a hospital, is also repeating his grade. Together, they play pranks on their friends and teachers and neglect their studies, thereby earning a notorious reputation as "problem"students. One day, Du-su goes to the house of a teacher named Baek Sang-do (Hah Myung-joong), who rents a room from a grocery store owner, for after-school lessons. There, he falls for In-suk (Kang Ju-hee), whose mother owns the grocery store. When a classmate named Ho-cheol tells on him to Mr. Baek, he decides to get Ho-cheol back. During class, he paints Ho-cheol's glasses red and yells, "Fire!" Ho-cheol ends up getting his glasses broken, and Du-su gets sent to the principal's office. When Ho-cheol fails to show up at school after that day, Du-su goes to his house. He sheds tears of remorse when he learns that Ho-cheol, whose family is too poor to afford a new pair of glasses for him, fell during his milk deliveries and injured his leg. Du-su takes on the deliveries in Ho-cheol's stead. He even takes notes in class for Ho-cheol, and begins to develop an interest in study. While continuing to run the milk delivery route in secret, he gets into a car accident and hurts his arm. Du-su's good deeds come to light because of the accident, winning him the approbation of his family and his teachers. Even In-suk, who had pretended to act coldly toward Du-su to make him come to his senses, visits him with a bouquet of flowers in her hand.