“I Saw the Devil” begins on a quiet snowy night when a young woman, Jang Joo-yun (Oh San-ha), has a flat tire on a deserted stretch of road. The school bus driver, Jang Kyung-chul (Choi Min-sik), approaches her, who offers assistance. She wisely refused but he broke into her car, beat her, and took her back to his terrible lair to dismember her body. Before she died, she revealed that she was pregnant and begged for her life.
Joo-yun’s body parts are soon discovered in a stream. Her father, Captain Jang (Jeon Gook-hwan), and her devastated fiance, National Intelligence agent Kim Soo-hyun (Lee Byung-hun), are at the crime scene. . Feeling guilty for not being there to protect her, Soo-hyun vows to take revenge on her killer.
Soo-hyun quickly lists the suspects for Kyung-chul. He visits the suspect’s separated parents and abandoned son to get contact information and goes to the killer’s ramshackle home. He finds Joo-yun’s engagement ring in the sewer of the torture chamber, proving that the bus driver was her killer.
Following Kyung-chul, Soo-hyun interrupts him as he prepares to sexually assault and kill a schoolgirl. After a fierce fight, Soo-hyun knocks the suspect unconscious and inserts a tracking device into his throat. The lawyer had no intention of arresting him or killing him on the spot. He plans revenge to turn Kyung-chul’s existence into a living hell. He released him and even offered a cash deposit to prolong the chase. It’s a dangerous game — Kyung-chul is a fierce and ruthless enemy, and it’s even more dangerous for innocent civilians caught in the crossfire.
