Evil lives inside.
Kyung-chul (Choi Min-sik) is a dangerous psychopath who kills people because of pleasure. He has committed a lot of terrible crimes against helpless victims all time avoiding to be arrested by the police. On a cold night, he found a new victim, Joo-yeon (Oh San-Ha), daughter of a retired inspector and the fiancée of a special secret agent Kim Soo-hyeon (Byung-hun Lee). After they find the body of Joo-yeon in horrific condition, Soo-hyeon obsessed with revenge, decides to himself start the hunt for the killer. He promised himself that will do anything to avenge his fiancée even if it means that will have to become the same monster as well as Kyung-chul.
I Saw the Devil is a very violent and cruel achievement by filmmaker Kim Jee-woon (The Good, The Bad, The Weird and A Tale of Two Sisters) which is not intended for everyone. Unlike most revenge thriller here are the two main characters constantly faced, causing immense pain to one another, each on its own way.
Byung-hun Lee and Min-sik Choi are great tandem that gives the whole project additional energy and makes it tense during the entire time. Director Kim Jee-woon has made another great movie that is brilliant and above all, unique.