A Japanese man has recently settled in a quaint village nestled within the South Korean mountains, residing in an isolated house. As a mysterious illness begins to sweep through the community, villagers experience a concerning rash that escalates into violent, murderous episodes followed by stupor and ultimately, death.
Amidst this chaos, police officer Jong-goo dives into the investigation, crossing paths with an enigmatic young woman named Moo-myeong, whose name translates to "no name" in Korean. She provides Jong-goo with unsettling insights about the Japanese stranger before vanishing without a trace. A local hunter shares a chilling account of sighting this nearly naked stranger consuming a deer, his eyes glowing ominously red. Jong-goo finds himself deeply disturbed, having had dreams of this individual as detailed by the hunter.
Determined to uncover the truth, Jong-goo recruits another police officer and a Japanese-speaking deacon to assist in their search for answers regarding the stranger. Together, they explore the stranger's home, only to stumble upon harrowing images of the infected villagers and their belongings. Overcome with shock, the other officer is unable to articulate what he saw. The situation escalates when the stranger’s guard dog violently attacks both the deacon and Jong-goo. Upon the stranger's arrival, the dog is calmed, prompting the trio to leave.
The plot thickens as Jong-goo discovers that a shoe belongs to his daughter, Hyo-jin, who soon succumbs to the same illness plaguing the villagers. In a fit of rage, he confronts the stranger but finds that all incriminating evidence has been destroyed. Consumed by anguish over Hyo-jin's deteriorating health, Jong-goo's mother-in-law seeks help from a shaman named Il-gwang. During the exorcism ritual, the stranger simultaneously conducts a ceremony of his own.
When Jong-goo finds Hyo-jin suffering, he interrupts the shaman’s efforts, opting to rush her to the hospital instead. The following day, he and his friends set out to hunt the stranger, believing he is the source of the affliction. However, during their pursuit, they are attacked by an infected villager. In a series of events that sees them hitting the stranger with their vehicle, they ultimately dispose of his body, all while Moo-myeong observes from a distance.
Despite having believed they had slain the threat, Jong-goo returns to find Hyo-jin’s condition seemingly improved. However, Il-gwang soon suspects that they might have inadvertently killed the wrong person. The shaman encounters Moo-myeong and starts to exhibit symptoms of illness himself, calling Jong-goo to warn him that she may be the true demon behind all the horror.
The tension heightens when Jong-goo finds Moo-myeong outside his home, who implores him to trust her intentions. She claims to have laid a trap for the demon but warns that it won't work if he returns home. Meanwhile, Il-gwang advises Jong-goo to be wary of Moo-myeong, insinuating her untrustworthiness. Torn between conflicting narratives, he ultimately departs to protect his family, but Moo-myeong’s appearance shifts, revealing ghostly skin as he notices she possesses items from the previous victims. As he breaks free from her grasp and steps over his home's threshold, the floral trap she set begins to die.
In a chilling turn, the deacon encounters the Japanese stranger alive and well in a cave, where he witnesses the stranger transform into a demon after he is photographed. Back home, Jong-goo discovers that Hyo-jin has committed murder within their family. When he calls out to her, she attacks him with a knife, plunging into a stupor afterward. Il-gwang arrives at their home and, while collecting evidence, accidentally drops a box containing photographs of other victims.
As Jong-goo lies on the brink of death, he is flooded with memories of happier moments with Hyo-jin, assuring her that he will always protect her, highlighting the tragic depths of their ordeal.
