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."

When a demonic uprising threatens to end the world as they know it, a monk and an excommunicated priest join forces to protect a powerful young boy and the next generation of magic. As dark forces close in, the unlikely trio must confront their inner demons.

To escape an arranged marriage in the Dragon Palace, Kim Yong-man, the dragon king's daughter, flees to the human world and begins high school, determined to become the top of her class. There, she encounters Shim Hae-soo, the current number one student sitting beside her. As rivalry shifts into friendship, the two gradually become special to each other.