Tucker & Dale are two good ole' boys, a little shy but inclined to be nice and neighborly, who are trying to enjoy a vacation in the mountains, when suddenly they find themselves confronted with a threat they never imagined--Yankee asshole college students! Just remember: These people are the "evil" part of the equation.
The kids imagine the titular hillbillies (now there's two words I never thought I'd use in that order) to be cannibalistic killers, and thinking themselves to be Bruce Campbell in Evil Dead types (the first Evil Dead, the one they tried to take seriously), rush in to "save the day" and end up taking themselves out one by one.
Of course, a good idea (Pixar, talking cars!) does not always a good film make (the actual movie Cars). And boy does this one tread into some dangerous areas, always a risk when you're juggling tones. But the cast and crew develop the idea so neatly that almost everything falls into place.
Only in literally the last shot of the movie, to my mind, does the movie hurt itself. Obviously I can't get into specifics but I can say, at least, that this one slip-up does not invalidate the whole movie.
Incidentally, the film is set in West Virginia of the United States but was made by Canadlians. You won't notice the discrepancy, though, since the Vancouver or Calgary-spawn are pretty good at disguising their northern origins (they do it so they can sneak over the border fence and take our jobs).