A violent storm hits Bridgton, Maine, and a tree crashes through the lakeside home of artist David Drayton, his wife Stephanie, and their eight-year-old son Billy. The next morning a strange mist is moving across the lake. David and Billy head to the supermarket with their neighbor, lawyer Brent Norton, and pass military convoys on the way.

Inside the store, a terrified man named Dan Miller runs in and warns that something in the mist is killing people. A siren sounds, and the mist swallows the building. One woman begs for someone to walk her home to her children. No one will, so she goes alone. Against David's advice, young bag boy Norm goes out to fix the generator, and tentacles drag him into the mist. Mrs. Carmody, a religious fanatic, begins preaching that this is God's punishment. Brent does not believe any of it and leaves with a small group, and they do not come back.

At night, giant flying insects swarm to the store's lights, followed by larger winged creatures that break through a window. People die in the attack, and when an insect lands on Mrs. Carmody without stinging her, she wins new followers. David's group makes a trip to the pharmacy next door for medicine. There they are attacked by huge spider creatures and find a military policeman caught in a web, who dies when tiny spiders burst out of his body.

Back at the store, Mrs. Carmody calls for a sacrifice. A young soldier, Private Jessup, admits that the mist came from a military experiment called the Arrowhead Project, which opened a door to another dimension. The crowd stabs him and throws him outside, where a creature takes him. The next morning, Mrs. Carmody demands Billy and Amanda too. Ollie shoots her dead, and David's group runs for his car. Ollie and others are killed on the way. David drives off with Billy, Amanda, Irene, and Dan.

David stops at his home and finds Stephanie dead, wrapped in spider webs. They drive on through the wreckage and see a giant creature walk over the road, so huge that it makes them feel like nothing. Then the car runs out of gas. There is no hope left, and there are five people in the car with only four bullets in the gun.

With no hope left, David shoots Billy, Amanda, Irene, and Dan, so that they will not be torn apart by the monsters. He has no bullet left for himself. He gets out of the car and screams for the creatures to come and kill him.

Something large comes out of the mist. It is an army tank. More tanks follow, along with soldiers using flamethrowers, and the mist begins to clear. David understands that if he had waited just a little longer, all of them would have lived, including his son. A truck full of survivors rolls past, and on it is the woman who left the store at the start to reach her children. She is alive and her children are with her. David collapses, screaming in anguish.

What does it mean? Fear killed David's family, not the monsters. He spent the film fighting the panic of others, then gave in to it himself at the worst moment. The woman on the truck is the final cruel point: she was the one who kept going, when no one, including David, would help her. Darabont said he sees The Mist as a companion to The Shawshank Redemption. One is about the value of hope, and the other is about the danger of losing it.

How is it different from the book? Stephen King's novella has no such ending. It simply stops, with the group driving on into the fog and a faint radio signal offering a small hope. Darabont wrote the new ending himself, and King backed it and praised it.