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.

Kyung-chul is a dangerous psychopath who kills for pleasure. He has committed infernal serial murders in diabolic ways that one cannot even imagine and his victims range from young women to even children. The police have chased him for a long time, but were unable to catch him. One day, Joo-yeon, daughter of a retired police chief becomes his prey and is found dead in a horrific state. Her fiance Soo-hyun, a top secret agent, decides to track down the murderer himself. He promises himself that he will do everything in his power to take bloody vengeance against the killer, even if it means that he must become a monster himself to get this monstrous and inhumane killer.