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.

Lee Su-hyun
as Lee So-ri (voice)

Kim Min-ju
as Park Dong-soon (voice)

Min Seung-woo
as Jung Ho-yeon (voice)

Nam Doh-hyeong
as Ahn Seung-kyu (voice)

Kim Yeon-woo
as Kim Ji-min (voice)

Lee Mi-na
as Kim Sunny / Lee Su-kyung (voice)

Kim Chae-rin
as Cho Ho-ran (voice)

Kim Yea-lim
as Yang Song-hee (voice)

Kim A-rong
as Young Lee So-ri (voice)

Lee Serena
as Young Jung Ho-yeon (voice)