The morning after a powerful thunderstorm in Bridgton, Maine, a thick mist descends on the small town. It is a real pea-souper and visibility is reduced to almost zero. The mist also hides bizarre and fantastical creatures within it; the creatures are dangerous and bloodthirsty and attack anyone who steps outside.
The local supermarket has become the hiding place for several locals who are trapped there while in the middle of their grocery shopping. Among them are David Drayton, a local commercial artist, and his son, Billy. They also brought a neighbor, Brent Norton, with them, as a car fell onto his car in the storm and he did not have any transport. There are others trapped too, including soldiers from the local military installation where strange goings-on have been reported, under the guise of "The Arrowhead Project".
The store's generator exhaust vent was plugged in after the storm. Nobody knows who did it. A young man who works packing groceries goes outside to see if he can unplug it, but he never returns, swept into the mist by a tentacled creature. After witnessing this, David tries to convince his fellow shoppers that whatever is lurking outside is a threat to their existence; even when the store manager, who also saw what happened to Norm, chimes in, some of the others think he is lying. They decide that the best thing to do is go for help, so they go outside but are quickly killed. A pterosaurus takes advantage of the open store doorway and comes into the store, killing some of the people inside.
The shoppers descend into accusations, paranoia, and fear that drives them out of their minds. Mrs Carmody, an extremely religious woman who is also stuck in the store, emerges as the group's leader. She convinces everyone that this is a Biblical event, a punishment for the bad acts of humanity. She tells them that they must make a human sacrifice to assuage God's wrath.
David and Ollie realize that they have to lead their allies in an attempt to return to some kind of sanity. They try to leave the market without being seen by Mrs. Carmody orders her followers to kill the sacrifices she has selected; one is David's son, Billy, and the other is a young woman named Amanda. Both are allies of David and Ollie. Ollie finds a gun in Amanda's purse and shoots Mrs Carmody dead. Her followers are suddenly in disarray, giving David and Ollie the chance to lead an escape attempt.
Ollie is cut in two by a giant clawed creature as he runs towards David's car. avid, Billy and Amanda reach the car safely and are joined by a feisty elderly schoolteacher, Hilda Reppler. They speed away from Bridgton and drive through the mist to New England. They stop for the night and listen to the radio but there is very little reception and David can hardly hear anything. He suddenly manages to make out the word Hartford, and feeling encouraged, begins to drive to Connecticut.
