Ka Ran and Nam Joon were classmates in elementary school. Ka Ran is a tomboy who prefers trousers to skirts, and loves taekwondo more than her piano lessons. Nam Joon on the other hand is Mr. Perfect and Wonderful who's got the good looks and the fine brain. No doubt, he is the most popular guy among the girls. One day, Ka Ran witnesses the scene where Nam Joon gives away the read moresweet bun which he got from Ka Ran's friend to another girl. Ka Ran throws herself at Nam Joon and beats him up. From then on, their "nightmare" begins; their life was connected since then and even after they have left their school.
They meet again when they are adults and right after breaking off with their other half. Arguments and conflicts still continued but then they realized that the other is the fated person in their life.