Academy-bus driver Jang Kyung-chul happens upon Jang Joo-yun one snowy night and offers to help fix her flat tire. Kyung-chul kills her and scatters her body parts. When a boy discovers one of Joo-yun's ears, the police are called in under the command of Section Chief Oh and Squad Chief Jang, the latter of whom is the father of Joo-yun. Kim Soo-hyun, a secret service agent of the National Intelligence Service (NIS) and Joo-yun's fiancé, vows to track down and take vengeance on Joo-yun's murderer.
Jang supplies Soo-hyun with a list of four suspects, including Kyung-chul. Searching Kyung-chul's home, Soo-hyun finds jewelry and underwear taken from (apparently) numerous victims. The discovery of Joo-yun's engagement ring proves that Kyung-chul is the killer. Soo-hyun puts a tracking device on the academy bus, following then attacking Kyung-chul while he is sexually assaulting his latest victim—one of the schoolgirls he was transporting home. Beating him unconscious, Soo-hyun places an NIS transmitter inside Kyung-chul, allowing him to track him on radar and listen to his conversations. Waking up, Kyung-chul flags down a taxi. During the ride, Kyung-chul realizes that the two men in the cab are thugs looking to rob and possibly kill him; he stabs both men to death. After finding the real taxi driver in the trunk, Kyung-chul disposes of all three bodies before going to a medical center, where he attempts to sexually assault a nurse Han Song-yi. Soo-hyun intervenes and slashes Kyung-chul's Achilles tendon before letting him go again.
Kyung-chul goes to the home of his friend Tae-joo, a cannibalistic murderer. After explaining his situation, Tae-joo remarks that whoever is after him must have some relation to one of his victims. Soo-hyun arrives, proceeding to incapacitate both murderers along with Tae-joo's girlfriend Se-jung. The next day, both Tae-joo and Se-jung are found by the police and sent to a hospital. A trusted subordinate of Soo-hyun's ensures he and Kyung-chul are sent to a private medical area away from the police. The barely conscious Kyung-chul hears them talk about the transmitter inside him.
Soo-hyun dumps Kyung-chul, intending to continue stalking him. Kyung-chul taunts him over the transmitter, now knowing who he is. Kyung-chul brutally assaults a store owner, forcing Soo-hyun to rush to the aid of the victim. Kyung-chul uses this time to defecate out the transmitter and place it inside a taxi driver he viciously assaults. Soo-hyun interrogates Tae-joo and learns that Kyung-chul is going after Joo-yun's father Jang and sister Jang Se-yun.
Soo-hyun arrives too late to stop Kyung-chul, who blinds Jang with a dumbbell and mutilates Se-yun. He abducts Kyung-chul before the latter can turn himself over to the police. Soo-hyun tortures him physically and mentally before setting up a guillotine above Kyung-chul's head that is soon activated when his parents and son arrive to visit and open the door to the room he is in. Placing a transmitter nearby, Soo-hyun hears the death of Kyung-chul and his family's reaction to his decapitated corpse. Alternating between sobbing and fits of laughter, Soo-hyun suffers a mental breakdown while walking away from the house.Lee Byung-hun as Kim Soo-hyun, an agent in the National Intelligence ServiceChoi Min-sik as Jang Kyung-chul, an academy bus driver and serial killerOh San-ha as Jang Joo-yun, Soo-hyun's fianceJeon Gook-hwan as Squad Chief Jang, Joo-yun's fatherKim Yoon-seo as Jang Se-yun, Joo-yun's sisterChun Ho-jin as Section Chief Oh, the leader of the policeChoi Moo-sung as Tae-joo, Kyung-chul's friendKim In-seo as Se-jung, Tae-joo's girlfriendYoon Chae-young as Han Song-yi, nurseNam Bo-ra as Section Chief Oh's daughter
The Korea Media Rating Board forced Kim to recut the film for its theatrical release, objecting to its violent content. The film received a "Restricted" rating twice, preventing any sort of release in theatres or on home video and promotions as well. Seven cuts were made with the total runtime of removed material between eighty and ninety seconds.
I Saw the Devil was released in South Korea on August 12, 2010. The film premiered at the 2011 Sundance Film Festival on 21 January 2011. It also received screenings at several other international film festivals, including the Fantasporto Film Festival, Toronto International Film Festival, Sitges Film Festival, San Sebastian Film Festival and the London Korean Film Festival.
American distribution rights were acquired by Magnet Releasing which released it in theatres on a limited basis on March 4, 2011.
I Saw the Devil received generally positive reviews from critics. Review aggregation website Rotten Tomatoes gave the film an 80% approval rating from critics based on 80 reviews and an average score of 7.1/10. Metacritic assigned the film a score of 67%, based on a weighted average of 19 reviews from mainstream critics.
Jeannette Catsoulis of The New York Times wrote, "From an unexpectedly moving first act to a hilariously disgusting sojourn with Kyung-chul’s cannibal pal, Mr. Kim and his cinematographer, Lee Mogae, retain complete control of the film’s fluctuating tones and impressive set pieces." Mark Olson of the Los Angeles Times wrote, "There is all the violent mayhem, for certain, but the thing that sets I Saw the Devil apart is its undercurrent of real emotion and how unrelentingly sad it can be." Rob Nelson from Variety magazine stated, "Repugnant content, grislier than the ugliest torture porn, ought to have made the film unwatchable, but it doesn't, simply because Kim's picture is so beautifully filmed, carefully structured and viscerally engaging." Horror website Bloody Disgusting gave it a 4.5/5 rating and stated, "I could talk for hours about I Saw the Devil, but nothing I can say will ever do it justice. The film is an experience; it’s something that will have you emotionally invested in the characters, while also covering your eyes at the extreme violence", whereas Empire awarded the film with a 4/5 score, stating, "This gleefully black horror-thriller is a very classy follow-up to The Good, the Bad, the Weird for Kim Jee-Woon". Phelim O'Neil from The Guardian wrote, "There's no shortage of Korean revenge-thrillers, but this, along with the recent The Man from Nowhere, proves there is plenty of life left in the genre" and gave it a four star rating out five.
Not all critics were favorable towards the film's brutality; Mark Jenkins of The Washington Post wrote, "Director Kim Jee-woon is a born filmmaker, even if this script (written by Park Hoon Jung and adapted by Kim) is unworthy of his efforts" and rated it 2 out of 5 stars. The Hollywood Reporter wrote, "On any number of levels, Devil is troublesome at best, offensive at worst".
Rolling Stone magazine put I Saw the Devil in the top 20 of "the scariest movies you've never seen".
The film was released on DVD in South Korea on 29 March 2011 as a 3 disc set, which contains both the Korean theatrical version and international version.
Special features include:Making OfArt DirectionAction FeaturetteCostume and MakeupSpecial MakeupInterview CollectionMusic FeaturetteAdditional Scenes with Director CommentaryPoster Shoot
It was released on DVD and Blu-ray Disc in the US on May 10, 2011. Special features include deleted scenes and "HDNet: A Look at I Saw the Devil".