After a violent storm, a dense cloud of mist envelops a small Maine town, trapping artist David Drayton and his five-year-old son in a local grocery store with other people. They soon discover that the mist conceals deadly horrors that threaten their lives, and worse, their sanity.

Everyone thinks of it as a harmless lightning storm. When Dave Drayton notices a strange mist on the lake, he thinks nothing of it. When he, his son, Billy Drayton, and his neighbor Brent Norton travel to the supermarket, the unthinkable happens. On their way to the market, they see the army, firefighters, and the police heading toward the mist. When he sees this Brent mentions something about "Project Arrowhead", a secret military project that no one knows about. As they are shopping, they see three soldiers walk in, pick up a few things, then head toward the mist. All eighty of the store's shoppers have no clue what is going on until an old man runs in the market with a bloody nose and declares "Something in the mist!" He tells them to close the door. About five seconds after they close the door, the entire store shakes, as though it has been lifted several feet above the ground. When David is asked to check on the generator, he finds the loading dock door being pushed on by ...