A forest-dwelling hermit discovers his dog has died from a mysterious illness and is sprayed by its infected blood. Five young friends, Paul, Karen, Bert, Jeff and Marcy have rented a cabin in the same woods for a week-long vacation. The group stops for supplies at Priddy’s General Store. Dennis, the intellectually-disabled son of the store manager, unexpectedly bites Paul’s hand.
Bert ventures into the woods with a rifle. Bert encounters and accidentally shoots the hermit. He panics when he realizes the hermit is infected with a disease and flees. He says nothing about the incident to his friends.
Later that evening, while the group hang out around a campfire, a stranger who calls himself Grim and his dog, Dr. Mambo, gatecrash their evening.
Later that night, the infected hermit comes to the cabin in search of help. Bert slams the door in his face and the desperate man tries to steal the group’s vehicle. A conflict ensues between the hermit and the five friends, during which they accidentally cripple their car and set fire to the hermit. He runs off into the woods, presumably to his death.
The next day, Jeff and Bert head out in search of help. They encounter a farmer who offers to help them, but hastily leave when they discover that the hermit is her cousin. Meanwhile, Deputy Winston arrives at the cabin to investigate reports of the previous night’s commotion. Paul explains things without mentioning the hermit’s presumed death. Winston promises to send a tow truck before departing.
Dr. Mambo returns to the cabin without Grim and seemingly infected. He threatens Paul and Bert, before Marcy scares him off with the rifle.
While Paul and Karen make out, they discover that Karen is infected with the flesh-eating virus and her leg has started to decay. Fearful of contagion, the others lock her in the shed. Paul leaves on foot to find help, but the only people he encounters chase him away, mistaking him for a peeping Tom.