Two romantic couples, consisting of four teenagers, decide to spend a weekend together in a remote cabin tucked away in the woods. Ellen Sandweiss and her boyfriend Bruce Campbell head into the trees for a picnic, while the other pair, Scott Spiegel and Mary Valenti, stay behind at the cabin and pass the time with a game of Monopoly.
During their lunch, Bruce informs Ellen that they’re camping on an Indian burial ground. Ellen is uneasy, but Bruce reassures her that as long as they don’t disturb the graves, they’ll be fine. He wanders off to explore and uncovers an ancient dagger belonging to the Native Americans, and despite his own warning, he pockets the blade.
After lunch, Ellen naps. When she wakes, Bruce is gone, and she ventures into the woods to find him. The sight she discovers is horrific: Bruce lies dead, mutilated with brutal knife wounds. A demonic presence seems to lurk in the trees, and Ellen bolts back toward the cabin, chased by unseen forces. She bursts through the door just as the entity closes in on her. She tells her friends that Bruce has been murdered, but Scotty dismisses it as a joke and sets off to find him anyway. Shelly and Ellen grow anxious as Scotty remains missing.
Shelly goes to search for the men, but a possessed Bruce attacks and strangles her, ultimately stabbing her in the neck and killing her with the dagger he found, moaning, "Join us."
Ellen confronts her possessed friend and flees into another room, arming herself with knives. Something tries to enter the room where she hides, and she blindly lashes out, accidentally stabbing Scotty who had just returned to the cabin. The possessed Bruce then launches his assault on Ellen, but she manages to trap him outside, stabbing his hand several times. The bleeding Scotty urges Ellen to check the cellar for the gun they brought along. Downstairs, she trips on a broken step and injures herself, but retrieves the pistol and goes back up to find that Scotty has been stabbed to death.
The demonic Bruce resumes his attack, and Ellen ultimately severs his hand to free herself from the dagger. She fights to flee but is brutally beaten as the demonized Bruce closes in. In a final, desperate move, she uses the dagger still clutched in Bruce’s severed hand to stab him. Blood erupts as Bruce collapses, and Ellen finishes the grisly task by swinging an axe to mutilate the body further.
The horror lingers as the body of Scotty suddenly rises and turns toward the exhausted Ellen, who remains overwhelmed by what she has witnessed. The screen cuts to black, leaving her fate—and the woods’ secret—unresolved.
