I Saw the Devil synopsis
On a dark road, taxi driver Kyung-chul (Min-sik Choi) comes across a scared female motorist stranded in a broken-down vehicle. He pulls over -- but not to help her. When the woman's head is discovered in a local river, her devastated fiancé, Kim Soo-hyeon (Byung-hun Lee), a trained secret agent, becomes obsessed with hunting down her killer. Once he finds Kyung-chul, things get twisted. After brutally beating the murderer, Kim lets him go free, and a demented game of cat and mouse begins.