The film takes place 10 years after the events of the third installment. A group of friends, including a young couple, Mike (Sean G. Krinn) and his girlfriend, Emma (Emma Greenwell), decide to take a detour on their way to a summer vacation. They stumble upon an old, abandoned mine in the Appalachian Mountains, which turns out to be the site of a series of gruesome murders.
As the group tries to survive and escape the mine, they are subjected to a series of gruesome and intense killings. The film features a high level of gore, with a focus on practical effects to create the violent and disturbing scenes.
The inbreds, who are known for their brutal and savage behavior, become the primary antagonists of the film. They are a group of deformed and twisted individuals who have been living in the mine for years, surviving on human flesh.
The mine, which was once the site of a horrific massacre, becomes the central location for the film's carnage. The group soon discovers that they are not alone and that a group of cannibalistic inbreds, led by Three Finger (Robert C. Treat), are still lurking in the mine.