A veteran female detective, So-Young, and her rookie assistant are investigating a recent series of bizarre and horrifying murders. All of the victims seem to have died due to an acid unleashed inside of their bodies. As the two detectives discover more about the case, they soon realize the serial killings may have something to do with a death that occurred ten years ago in a salt house. Before long, So-Young becomes plagued by nightmares in which the dead girl appears and the horrific killings continue to confound.