Kim Hyun Ah was a 17 years old highschool girl. She had a big crush on one of her school mates, Changmin. One day, being already in a senior year, she had decided to do something brave. It was to make a confession. So she wrote a love letter and planned to give it directly to her crush. However, things just got complicated when she gave it to the wrong person who turned out to be Yesung, the delinquent. What made it worst was that Yesung then played along.