Revenge tales are tricky to pull off and even more difficult to lift above formula. By and large, they all follow the same basic blueprint, end up in the same place, and usually misjudge or overlook the story's moral implications. Director Kim Ji-woon, responsible for 2004's skin-crawling supernatural mystery "A Tale of Two Sisters" (remade for the U.S. in 2009 as "The Uninvited"), is aggressive and resourceful enough to play with expectations. "I Saw the Devil" is ceaseless in its goals to devastate, infuriate, nauseate, and finally challenge (in roughly that order) audiences used to more pandering, watered-down treatments of tragedy and vengeance.

The careful attention to detail is apparent from the startling prologue. Stranded alongside the road with tire trouble, the lovely Joo-yeon (Oh San-ha) is approached by Kyung-chul (Choi Min-sik), who at first looks like he wants to help. As Joo-yeon sits in her automobile, her fluffy, white steering wheel cover, mirroring the blanket of snow outside, proves a subliminal sign of her false security. She might as well not even be sitting behind a pane of glass. Before she knows it, Joo-yeon has been bashed over the head with a hammer and is faintly coming-to in a grimy warehouse. She pleads for her life and the life of her unborn child, but it's no use reasoning with a deranged psychopath. The next time she is glimpsed, she is but a pile of dismembered body parts. Suddenly, before any proper plot has been introduced, the viewer is already ruminating on a deeper level than the norm about how precious and profound life is and how quickly and easily it can be snuffed out.

When an ear is found in a field beside an overpass, it is only a matter of time before Joo-yeon's decapitated head is recovered from a nearby riverbed. Her detective father, Squad Chief Jang (Jeon Gook-hwan), is devastated, remarking to her grief-stricken fiance Soo-hyeon (Lee Byung-hun) how he has been professionally solving crimes for thirty years, but couldn't even save his own daughter. Using the skills he's picked up working as a special agent, Soo-hyeon embarks on a cat-and-mouse game with Kyung-chul that traverses much more than just the final goal of killing him. No, before he does that, he wants to destroy every facet of his life and give him a glimpse at the well-deserved hell he has brought upon himself.

After Soo-hyeon asks for a two-week leave of absence and sets out on his blazing path of insatiable payback, "I Saw the Devil" carries on for two more hours of uncompromising brutality that rarely leavens. They first come face to face while Kyung-chul is about to rape and murder his next victim, a school girl he has kidnapped from the after-school bus he drives and taken to a desolate nursery. Soo-hyeon beats him to a pulp, breaks his wrist, and has him unknowingly swallow a small GPS device before letting him go. When he pulls himself out of unconsciousness, Kyung-chul can't believe he's survived and foolishly believes he's escaped. Not so fast. Soo-hyeon returns again just as the killer is about to attack a nurse, this time focusing his wrath on his Achilles tendon. These two encounters are only the beginning of what becomes a hunting game for Soo-hyeon. Once Kyung-chul has figured out his pattern, the stakes raise again. He's more than willing to play along with his pursuer.

Written by Park Hoon-jung and directed with twisted glee by Kim Ji-woon, "I Saw the Devil" flirts with exploitation, but never crosses the line (it crosses it plenty of other ways, though). Not simply a film that gets its kicks from the depravity it portrays, Ji-woon instead uses violence and the increasingly out-of-control actions of Soo-hyeon to comment on how easy it is for the line between good and evil, justice and criminality to become blurred. Yes, Kyung-chul deserves what is coming to him, but at what point will Soo-hyeon completely lose sight of the reason he wants to destroy this man little by little? Who else might he have to sacrifice to fulfill his blood mission? Is he in perilous danger of becoming no better than the killer he hunts? By the end, the innocent, ill-fated Joo-yeon is a distant memory—an epiphany that rushes over Soo-hyeon in a sudden, despairing burst. It is in that moment that he realizes he not only lost his future wife, but himself, as well.

"I Saw the Devil" is a crackerjack thriller, present and in-the-moment and with its fair share of surprises up its sleeve. It is also not without its contrivances. Soo-hyeon's finding of Joo-yeon's ring in a drain within Kyung-chul's lair is far-fetched and patly handled, a furthering of the narrative at the expense of plausibility. Additionally perplexing is the nightmarish snapshot of Korea as a place filled with murderers and deviants, extending beyond Kyung-chul to also include a serial killer friend of his, a woman whose own rape gradually turns her on, and two deadly thieves he coincidentally hitches a ride with at one point. These trouble areas aside, the picture is supremely well-made, and in Lee Byung-hun (2009's "G.I. Joe: The Rise of Cobra"), as Soo-hyeon, and the chilling Choi Min-sik, as Kyung-chul, are two performances of stunning commitment and effectiveness. "I Saw the Devil" is a title that refers not to a literal figure of malevolence, but to the dark nature in each of us. Soo-hyeon ultimately recognizes this in his own doing. He knows he must finish what he started, but isn't so sure who he'll be once he's reached the other side.