After a Japanese man arrives at Gokseong, a small village in the mountains of South Korea, a mysterious infection breaks out and causes the villagers to become deranged and violently kill their families.One night at the police station, officers Oh Seong-bok and Jong-goo are discussing the Japanese stranger when a naked woman appears in the rain. They later discover the woman, infected, has murdered her family. At the crime scene, Jong-goo meets a mysterious young woman called Moo-myeong who tells him the Japanese man is a ghost and the culprit. Jong-goo steps outside to call Oh Seong-bok, and the woman vanishes. A local hunter tells them he saw the stranger with glowing red eyes, eating a raw deer in the forest.Jong-goo has similar dreams about the stranger and decides to investigate with Oh Seong-bok. They enlist the help of Oh Seong-bok's nephew, a Japanese-speaking deacon named Yang I-sam. They investigate the stranger's house when he's out and find pictures of the murdered villagers and their belongings, as well as a worship room. The stranger's guard dog attacks them and stops when the stranger returns. Jong-goo and his team leave. On the way back, Oh Seong-bok shows his partner a shoe that belongs to Jong-goo's daughter, Hyo-jin. Hyo-jin grows sick, with symptoms similar to those of the other infected. Jong-goo returns to the stranger's house, but finds the evidence has been burned. Infuriated, he smashes up the worship room, kills the guard dog, and orders the stranger to leave the village.
Distraught about Hyo-jin's change in health and behavior, Jong-goo's mother-in-law seeks help from a shaman
Il-gwang. Il-gwang says a wicked spirit has possessed Hyo-jin. His exorcism fails. After learning Jong-goo disturbed the stranger, who he says is a demon, Il-gwang prepares a death-hex ritual and tells Jong-goo he must not be interrupted. At home, the stranger performs a ritual at the same time. Both he and Hyo-jin experience excruciating pain as Il-gwang's ritual progresses. Jong-goo stops the ritual and takes his daughter to the hospital instead. The stranger pulls himself into bed and sees Moo-myeong outside his house.
The following day, Jong-goo gathers his companions to hunt down the stranger. As they search at his house, they are attacked by one of the infected villagers, who injures Yang I-sam and gives the stranger time to flee. They pursue the stranger but lose him at a cliff. The stranger, hiding just out of view, sees Moo-myeong staring at him from afar. As the group drives back down the mountain, the stranger lands on the windshield. They dump his body off the cliff as Moo-myeong watches from above. Jong-goo returns to find Hyo-jin has seemingly improved.
Il-gwang drives to Jong-goo's, where he encounters Moo-myeong and vomits blood. Il-gwang flees town but then turns back and calls Jong-goo to say Moo-myeong is the real demon and the stranger was a shaman trying to stop her. Meanwhile, the wounded Yang I-sam receives news that Oh Seong-bok has killed his family. Hyo-jin disappears. While searching for her, Jong-goo meets Moo-myeong, who claims the stranger is still alive and she has set a trap for him, but it will fail if Jong-goo goes home, and Hyo-jin will kill everyone. Il-gwang calls Jong-goo again and tells him he should go home to protect his family. Moo-myeong says Il-gwang is a mere pawn of the real demon. Confused, Jong-goo asks her if she is a human or a ghost. She gives a cryptic answer. Jong-goo notices she is wearing the personal items of the victims, including his daughter's hair pin. Believing this to be proof she is responsible, he dashes home. The moment he crosses his threshold, the floral trap set by Moo-myeong withers.Yang I-sam returns to the stranger's house with a sickle and a cross.
He finds a secret cave nearby and the stranger alive inside. He asks the stranger about his true form and says he thinks the stranger is the devil, but if he's wrong he'll leave him in peace. The stranger laughs maniacally and says he is the one who won't let him leave. He photographs Yang I-sam while asking why he still doubts the stranger's identity. As Yang I-sam stands frozen, the stranger changes into a demon.
Back home, Jong-goo finds Hyo-jin has brutally murdered their family. He tearfully calls out to her, but she doesn't respond. Il-gwang arrives and takes photographs of Jong-goo's dead family. Returning to his car, he unintentionally drops a box filled with photos of the murdered villagers that the stranger claimed to have burned. As Jong-goo sits devastated by his daughter's death, he remembers happier times with his daughter and begins to smile, assuring her that he will protect her.
