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. "If you want to keep reading, find my second 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. "I want to find the last letter and thank you in person."