One day, 25 years ago, missing Chun-bae's letter is delivered to his cousin, Jjang-goo. The letter contains Chun-bae's lover, Bok-soon, and other people's letters. Jjang-goo finds Bok-soon to deliver the letter and meets class president Jang together. The three people recall their memories from 25 years ago, and eventually faced the person who sent the letter. Through that, the tangles begin to unravel.
| Actor | Role |
|---|---|
 Seo Hyun-wooJjang-gu | Jjang-gu |
 Han Sa-myeongChoon-bae | Choon-bae |
 Seo Jin-wonDetective Cha | Detective Cha |
 Yoo Ji-yeonBok-soon | Bok-soon |
 Kim Dae-jinChief Jang | Chief Jang |
 Im Ho-kyoungTeacher Kim | Teacher Kim |
| NNo Si-hongYoon-bae | Yoon-bae |
 Jo Young-jiYoung Ok-hee | Young Ok-hee |
 Lee Geun-hooChief inspector | Chief inspector |
| KKim Do-damGang | Gang |