Five old friends from university--Phil, Dom, Hutch, Luke, and Rob--meet over drinks at a pub, and discuss plans for a group trip to catch up with one another. Rob suggests hiking in Sweden, but is quickly shot down by the others. Later in the evening, the group exits the pub and passes a shop. Luke wishes to stop and purchase a bottle of Vodka, which Rob accompanies him in doing. While conversing in the shop, the pair notices the distressed cashier slumped against the wall behind the register. Two armed robbers emerge from the back room, and Luke hides behind a shelf at the end of the aisle, leaving Rob frozen in fear. The robbers antagonize Rob, demanding his wallet, watch, and ring. Rob parts with the first two willingly, but refuses to give them his ring, as it is his wedding band. Luke, still hidden from sight, flips the bottle in his hand and prepares to intervene. However, he is too slow to act, and Rob is bludgeoned to death before his eyes.
To honor Rob's wish, the four embark on a hiking trip along Kungsleden, or King's Trail, in Sarek National Park in northern Sweden six months after his death. When Dom loses his footing and injures his knee, impairing his ability to walk, Hutch consults the map and decides that an alternate route through a forest off the trail will take them half the time. Upon entering the forest, the group encounters strange phenomena, including a gutted elk hanging from the tree branches and strange symbols carved on the trees.
As night falls, a torrential rainstorm soaks the men. While looking for shelter, they come upon an abandoned cabin and decide to break in and stay overnight. Inside the cabin, they find necklaces hanging from the walls that depict similar symbols as the runes carved in the tree. While exploring the second floor of the cabin, Phil discovers a strange wooden effigy that is shaped like a decapitated human torso with antlers for hands.
During the night, the four are plagued by nightmares. Upon waking the next morning, Luke finds that he has sustained a set of strange puncture wounds on his chest. The group finds Phil in the attic, naked and kneeling in prayer in front of the effigy.
The group leaves the cabin to continue their travels deeper into the woods, trying to find a way out.
Upon climbing a ridge in order to gain some perspective regarding their location, Luke sees a human-like hand wrapped around the trunk of a distant tree. While trying to discuss his findings with the group, Dom is doubtful of his report. An argument ensues wherein Dom reveals that he blames Luke for Rob's death, and calls Luke cowardly for failing to act during the robbery.
Later that night, Luke is woken from another nightmare by screams. Discovering that Hutch's tent is empty, the three rush deeper into the woods, following Hutch's screams. By dawn, they realize that they have become lost and cannot recall where their campsite was. They decide to continue their search without their tents and supplies. The three come upon Hutch impaled on tree branches, much like the gutted deer they had found earlier. The group retrieves Hutch's body for his compass and knife, and then give him an impromptu burial by covering his body with tree branches.
Luke leaves Phil and Dom on the lower part of a ridge, climbing a hill that provides an overview of the entire forest. He realizes that they are relatively close to the edge of the forest, and also spots smoke rising from distant campfires. He rejoins the two to tell them the news, but finds them with their flashlights pointed towards the trees, saying they heard a noise. Suddenly, Phil is dragged off-screen by an unseen creature. Upon seeking a hiding spot, Luke encounters Dom, and urges him to run with him. They get to their feet and begin to run, the creature giving chase. They pass Phil's body impaled on the branches of a tree near a path of torches that leads to a small village. They seek shelter in the first building they seen, and collapse on the cabin floor, only to be beaten unconscious by its occupants.
When they awaken, they find themselves restrained by their hands in a basement. An elderly woman enters the basement and inspects the puncture marks on Luke's chest. She pulls down her dress to reveal a similar pattern on her chest. She turns to leave the basement and utters a command in a foreign language, which prompts two men to grab Dom and bring him to the upper floor of the cabin. A younger woman enters the basement and explains that preparations are being made for sacrifice. Some time later, Dom is escorted back to the basement, beaten and bloodied, but still alive. He explains to Luke that he will serve as a human sacrifice to the creature, and instructs him to find a way to escape and destroy the village.
Dom is taken outside of the cabin and brought to a wooden post, where his hands are tied behind his back. As day shifts to night, a roar can be heard from the forest. The captors immediately fall to their knees in worship. Dom has a vision of his wife emerging from the trees and holding his face in her hands. This is in reality the creature that has been pursuing the men, and it bears a resemblance to the wooden effigy they discovered in the abandoned cabin. The creature removes Dom from the post and impales him on the branches of a nearby tree, leaving him to die. Desperate to escape, Luke breaks from one of his restraints by breaking his thumb, but cannot undo the second one before he is interrupted by the young woman's sudden entrance. When Luke asks about the creature, she explains that it is called a Jötunn, a god-like figure stemming from Scandinavian mythology, and that they provide it sacrifices in return for immortality. She states that Luke will take part in a ritual where he will submit to the creature and join the cult, or be killed.
After she leaves, Luke breaks free from his restraints and leaves the basement. He ventures to the upper floor of the cabin, hearing prayers and screaming coming from behind a closed door. Armed with a burning torch, he opens the door and finds a twisted congregation of mummified, but still living humans, evidently the end result of the immortality granted by their worship of the creature. Following Dom's last wish, he sets the worshipers alight. This act attracts the Jötunn, who emerges from the forest to find the cabin burning. In a rage, the creature kills the remaining worshipers. Luke uses this opportunity to escape from the burning cabin undetected, armed with a rifle and an axe. Before running into the woods, Luke aims and takes a shot at the creature as it is about to kill a worshiper. The creature pursues him, attempting to cripple his mind by causing hallucinations of his recurring nightmare. The creature eventually catches him, and forces him onto his knees multiple times, offering Luke a chance to submit. He uses an axe which he had previously taken from one of the worshipers to strike the creature on its head. He then follows the sunlight, emerging from the forest into an open field. The creature does not emerge from the treeline, as it seems unable to follow. The monster roars at him, and he screams back in triumph. Luke turns from the monster and heads in the direction of a paved road with a passing car, a sign of civilization.