When their plans for a nature trip go awry, Polly and boyfriend Seth decide to check into a motel. On their way, they're carjacked and kidnapped by low-rent crooks Dennis and Lacey, who take the victims and their SUV to a nearby gas station. Along the way, they encounter an increasingly terrifying horde of parasites, and if any of them intend to survive, they'll have to outsmart the deadly organisms.