After summer vacation, So-ri transfers to a new school and finds an anonymous letter in her desk drawer with an introduction to the school and a clue to finding the next letter. While following the clues in the letters and searching all over the school like on a treasure hunt, So-ri keeps running into a same-aged student named Dong-soon. So-ri and Dong-soon quickly become friends as they search for the letters together. As she finds one letter after another, the special connection continues, and So-ri's curiosity grows for the mysterious sender.
