악마를 보았다

I Saw the Devil opens on a snowy night. Jang Joo-yun, pregnant and stranded with a flat tire, calls her fiancé, NIS agent Kim Soo-hyun, who tells her to wait for a tow truck. A school bus driver, Jang Kyung-chul, pulls up. When she refuses his help he smashes her car window, knocks her out with a hammer, and drags her to his lair, where he tortures, rapes, and dismembers her despite her pleas. Her body parts surface in a stream. At the recovery site Soo-hyun and her father, Squad Chief Jang, watch her severed head roll out of a bag. Soo-hyun vows to find the killer and inflict something worse than death.

Using NIS contacts, Soo-hyun gets a short list of likely suspects and works through them himself, brutally interrogating two before finding his fiancée's engagement ring at the home of Jang Kyung-chul. He locates Kyung-chul mid-assault on a schoolgirl, ambushes him, smashes his head against rocks, surgically implants a GPS tracker inside him, and lets him go — choosing to stalk and break him in stages rather than make an arrest.

The film becomes a cat-and-mouse inversion of a chase thriller. Kyung-chul wakes injured, hails a taxi, and butchers the two passengers who plan to rob him. Soo-hyun catches up, beats him again, and disappears. Every time Kyung-chul tries to resume his predatory life Soo-hyun appears at the last moment, maims him, and lets him crawl on. He listens through an earpiece, intervening for maximum cruelty rather than rescue.

Wounded and paranoid, Kyung-chul takes refuge with Tae-joo, an old acquaintance who is himself a serial killer and cannibal living with an abused girlfriend. Tae-joo recognizes that Kyung-chul is being followed and taunts him. Soo-hyun tracks them to the house and turns it into another battleground. By now Soo-hyun is mutilating without remorse; his colleagues realize he has gone rogue but cannot easily stop a trained agent operating off-grid.

Kyung-chul figures out a loved one of his victim is behind the torment. He goes to the home of Joo-yun's father and sister to inflict the same horror in retaliation. Soo-hyun arrives in time to subdue him, but the attack makes clear his crusade has put everyone he loves at greater risk.

For the final act, Soo-hyun does not hand Kyung-chul over to police. He transports him to his estranged parents' home, binds and gags him in a chair, and rigs a guillotine to the front door. When Kyung-chul's mother and young son arrive, they hear muffled thrashing but cannot understand him; they open the door and watch him decapitated in front of them. Listening through his earpiece from the road outside, Soo-hyun breaks down in tears — having become exactly the kind of monster he set out to destroy.