Hae-Joon, a seasoned detective, investigates the suspicious death of a man on a mountaintop. Soon, he begins to suspect Seo-rae, the deceased’s wife, while being unsettled by his attraction to her.

The film is based on a true yet unsolved murder case from the early 1990s that is known among Koreans as the “disappearance of the frog children.” In 1991, five elementary school students told their parents that they were going to hang out on a nearby mountain to catch frogs, and then they go missing.

A 5 member gang, led by Seok-Tae (Kim Yun-Seok), kidnaps a baby boy named Hwa-Yi and raises the baby like their own son. The baby boy is now 17 years old (Yeo Jin-Goo) and has turned into a lethal killer. Taking part in his father’s gang, Hwa-Yi learns about his own past. Hwa-Yi pulls out his gun to find out who he really is.

On a live music radio show, a letter arrives from the 23 years in the past. Through the letter, the first love and friendships of five friends are revealed.