Lee Su-hyun
Lee So-ri (voice)
Kim Min-ju
Park Dong-soon (voice)
Min Seung-woo
Jung Ho-yeon (voice)
Nam Doh-hyeong
Ahn Seung-kyu (voice)
Kim Yeon-woo
Kim Ji-min (voice)
Lee Mi-na
Kim Sunny / Lee Su-kyung (voice)
Kim Chae-rin
Cho Ho-ran (voice)
Kim Yea-lim
Yang Song-hee (voice)
Kim A-rong
Young Lee So-ri (voice)
Lee Serena
Young Jung Ho-yeon (voice)
Park Ki-wook
Lee So-ri's Father (voice)
Kim Soon-mi
Doctor (voice)
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.
