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 vengeance when his fiancée gets brutally murdered by psychopathic serial killer Jang Kyung-chul (Choi).

Plot

Jang Kyung-chul, a school-bus driver, encounters a woman named Jang Joo-yun and offers to fix her flat tire. After beating her unconscious and bringing her to his house, Kyung-chul methodically dismembers her, unaware of her ring falling into a 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, and Squad Chief Oh. Joo-yun's fiancée Kim Soo-hyun, an NIS agent, vows to exact vengeance against the perpetrator.