Boeun is an ordinary high school girl who worries about her study and gets a crush on baseball team ace at school. One day, her grandfather orders her and Sangmin, to get married. Sangmin, who grow up with Boeun and now an university student, always makes fun of Boeun and treats her like a little sister. Although both of them object this marriage, their undercover marriage life begins and Boeun even starts dating with the team ace. It seems that everything goes smooth until Sangmin is assigned to Boeun's school as a pupil teacher...