I Saw the Devil is a 2010 South Korean action-thriller film directed by Kim Jee-woon and written by Park Hoon-jung. Starring Lee Byung-hun and Choi Min-sik, the film follows NIS agent Kim Soo-hyun (Lee), who embarks on a quest for revenge when his fiancée is brutally murdered by the psychopathic serial killer Jang Kyung-chul (Choi).

Plot

A school-bus driver named Jang Kyung-chul encounters a woman named Jang Joo-yun and offers to fix her flat tire. After beating her unconscious and bringing her to his home, Kyung-chul methodically dismembers her, unaware of her ring dropping and getting stuck in the floor drain. Kyung-chul scatters the body parts into a local stream, where it is discovered, prompting the police to conduct a search led by Squad Chief Jang, Joo-yun's father. Her fiancé, Kim Soo-hyun, an NIS agent, vows to take revenge against the murderer.