Na Du-su, who sports the nickname of "Yalkae" (meaning "cheeky, cocky person" ), has been in second year in high school for two years. His partner-in-mischief Yong-ho, 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, who rents a room from a grocery store owner, for after-school lessons. There, he falls for In-suk, 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....