An alien pod crash lands in the swamps, after which the local mosquitoes begin feeding on the alien corpses. This causes the mosquitoes to grow to massive sizes and develop an unquenchable hunger for blood. A mass swarm of these mutant insects attacks a Michigan national park in a brutal onslaught that kills the entire tourist population. The few unrelated survivors - two lovers, a meteorologist, a punk rock musician, and a gang of dangerous bank robbers on the run from police - must settle their differences and band together before nightfall, when the nocturnal mosquito swarm will return for more blood.