The hunter Johannes Brost as Gunnar is looking for his daughter Lisa. He finds her lying on the ground, being eaten alive by his wife. Gunnar shoots his wife twice in the head, killing her. The opening titles include rough sketches, showing the history of the Vættr.
The young couple Patrik Almkvist as Albin and Lisa Henni as Ida are planning a weekend trip to a remote cabin with Ida's brother, Patrick Saxe as Simon, and a couple of friends. The group drive to a remote woodland area and are forced to walk the last bit. While walking to the cabin, the group notices Gunnar watching them from a cliff and the aggressive Simon shouts him off. The group finds the cabin locked, contrary to the information given by Albin's father, Olof. While Albin tries to pick the lock on the front door, his friend Marcus finds an open window on the backside and talks Jessica Blomkvist as Marie into climbing in to scare the others. Marie climbs in and goes into the basement to investigate, finding an axe. She does not notice an earth-like creature watching her from the shadows.
The group finally gets into the cabin and is scared by Marie's sudden appearance. She does not seem to be well. While the others install themselves on the ground floor, Ida's goth-friend Tove brings Amanda Renberg as Linnea upstairs and instigates a sexual encounter with Simon, despite Linnea's love for Simon. The group throws a party, but Marie's sickness gets worse and she attacks Tove, biting off her lip, and then spits blood in Linnea's eyes. Simon captures and binds Marie. Gunnar arrives at the cabin, suggesting the group should kill Marie. He tells them about the Vættr and how his own family were killed in the cabin just a few days prior. Gunnar informs the group that the Vættr have the ability to steal the souls of the living by looking in their eyes, turning them into the undead. Gunnar also warns them that the undead slaves of the Vættr are very infectious. While Marcus tends to Tove she is resurrected and attacks Marcus, wounding him. Gunnar arrives and saves Marcus but Tove manages to escape.
Simon and Linnea arm themselves with shovels in the shed and go to fetch police aid which Albin was called for. Gunnar goes out to kill Tove, but she ambushes and bites him. Gunnar proceeds to beat Tove into a pulp before carving off her head with his fishing knife. Gunnar then commits suicide in the shed. Simon notices infection in Linnea's eyes and beats her unconscious and buries her alive. Overcome with grief over his action, he ignores his instruction and goes back to the cabin to save his sister. The aid of armed lawmen is now impossible. Meanwhile, Marie breaks free and attacks the others. Albin, Ida and Marcus fight her but Marcus dies before Albin crushes her head with a rock. Marcus turns into an undead and Albin and Ida hide upstairs, unable to escape. Simon arrives and tries to help them but is thrown from a window by Marcus. Simon survives but is attacked by an undead Linnea. Albin uses Ida's cellphone to determine that they can leave their hideout and Albin grabs the axe Simon had fetched before but dropped. Marcus and Linnea attacks, but Albin kills Linnea with the axe before Marcus takes it from him. Albin and Ida lock themselves into a room and dig through the floor, dropping in the kitchen. Albin breaks his arm in the process. Marcus jumps after him, but Albin pushes a table beneath the hole, which Marcus impales himself on. Albin then kills him with a hammer. Simon attacks Ida and forces his blood into her mouth with a kiss. Albin is unable to save her and rushes to the shed and grabs Gunnar's rifle and manages to blow off Simon's head with a point-blank gun blast.
Albin returns but realises Ida is undead too. Ida is somehow able to remember Albin and hesitates in attacking him. After much hesitation, Albin kills her by dropping a heavy shelf on her. Albin cries over his losses, but suddenly there is movement beneath the floorboards. The Vættr herself ascends from the basement and inspects the devastation. However, Albin closes his eyes and turns off the lights, rendering the ancient and fragile creature powerless. Albin drops the fridge on the Vættr, crushing it. Albin then leaves the cabin, now having become the "new Gunnar".
