Twenty-two year-old serial killer Shin Hyun targeted pregnant women and turned himself in after committing several grisly murders. Ten months later a copycat killer became active, and detectives Kang and Kim are put on the case.
They track down the killer by following clues from the first two victims: A pregnant schoolgirl whose fetus the killer removed, and a single mother strangled from behind on a bus. They had tried to pump Shin for information, but without success. They stake out the killer's home, but when the killer comes home, he notices the cops and runs. Kang follows him into a nightclub, where the killer slices off a lesbian's ear and then slits her throat, just like Shin's third victim. Kang fires four rounds into his chest; this puts him in a coma. Despite this, the murders still continue. The police capture the next suspect, but the murders keep occurring.
In the end, it is revealed that Shin's mother had tried to abort him, but he survived. The psychologist Dr. Chu had used post-hypnotic suggestion on the other two killers to make them commit copycat murders. Kang then kills Chu (imitating Shin's murder of an abortion doctor), but Kang is triggered by a CD mailed to him. Kang kills his own mother, who was a prostitute.
In the course of the movie, Shin is executed and his final words are "I killed my mother." This explains the sixth and last unidentified victim, whom he brought in a bag to the police station when he confessed. Kang is about to commit suicide when Kim shows up and kills him instead.
Just before the ending credits, the letter "H" appears and expands into the word "hypnosis", and its definition.