David and Steph live off a lake in Maine with their five year-old son, Billy. The summer had been hot and a few storms have rolled in, but David thinks they will need to go to the basement for this one. They watch the lightning for a while until a water cyclone starts at the other side of the lake. David sends his family to the basement and goes to look for candles. The power goes out when he finds them. They end up downstairs in the guest room for the night. David dreams of a giant God walking around the lake and a mist like smoke covering everything.
The next day, David surveys the damage. He tells Billy to stay away from the fallen power lines that were ripped away from their house. He goes to get tools to remove the fallen trees from the driveway. Steph admits she is scared, but David thinks the old Antiquary owner, Mrs. Carmody, has been telling her too many stories. Billy drags his parents around the house to show them the damage. Norton’s (neighbor they don’t get along with due to a property dispute law suit) dead pine tree landed on their boathouse. Billy goes to fetch the flag off the pole that fell into the lake. That is when they see the other side of the lake, a town called Harrison, is completely covered in white mist. David tries to reassure his family that it is nothing to be concerned about. Billy goes to the Bibber’s to see their damage. David hears Norton swearing over his fancy chainsaw and smiles before starting his own cheap one. Norton comes by later to ask to borrow David’s car to go to town because his was smashed under a tree. David offers to take him with him and Billy when they go. He also tries to convince Steph to go when he sees the mist has gotten closer. She refuses because of Norton and they leave with Steph waving in the garden. David sends Billy into the store with Norton to start the grocery shopping while he tries to call Steph. The payphone isn’t working so he goes inside. David sends Norton to get his things so they can get through the line quicker. He tells Billy he isn’t worried, but he really is.
David, Billy, and Norton wait in one of two extremely long check out lines (cashiers doing everything manually and generators running refrigeration units only). They hear sirens go by and a teenager comes in talking about the fog that is rolling in. Another man comes in with a bloody nose and says something in the mist took another man. Everyone starts cramming towards the door to see what is going on. The mist comes in and the store gets darker. Norton starts to suggest they leave when there is a thud. He tries to calm everyone down and David backs him up. Mrs. Carmody pipes in saying the mist is death. A woman asks for anyone to take her home to her kids who are alone. Everyone refuses and she walks out the door.
David takes Billy back by the meat counter because the boy is crying and upsetting people. He lets his son cry himself to sleep and then goes into the storage room to find him a blanket. He notices the generator’s exhaust is pouring back into the room rather than outside, so he shuts it off. The lights go out and David only has a sliver of light to find his way out. He hears slithering outside. Ollie (manager), Norm (bag boy), Jim and Myron arrive with flashlights and he tells them what happened. They go in and decide to turn the generator on long enough to open the loading door and remove whatever is blocking the exhaust from going outside. Against David’s pleas, they send Norm outside. He is grabbed by a giant tentacle that starts sucking the life out of him. David tires to pull him back in, but gives up when he realizes he has to live for Billy. He grabs a broom and is able to hit the button to close the door. Myron tries to apologize, but David hits him until Ollie makes him stop. David and Ollie decide they have to tell people to keep them from trying to go outside. David finds a blanket for Billy.
David tells Norton what happened, hoping the man will be the right person to calmly explain to everyone. Norton refuses to believe him and starts making a scene about how they are all playing a joke on him because he is only a seasonal resident. Bud (manager) and everyone else is gathered, but no one believes David except Mrs. Carmody. David takes Bud into the backroom, finds a flashlight, and shows him a piece of tentacle that was cut off by the door closing. Bud Finally believes.
A man named Miller gathers people around to get ideas on how to make the store safe for everyone. They cover up the front windows and leave five “loopholes” that they rotate guards on. Norton insists to be let out of the store with his four followers. David asks him to tie a 300 foot clothes line to him and tie it to something so they know they made it at least that far. Norton refuses, but another man in his group agrees. David lets out the rope as the group travels. When there is only a few feet left, the rope starts to go wild and there is a scream. They hear the old lady that went out with them yelling for help, but she is silenced quickly. David pulls in the rope to find it shredded and bloody at the end. They close the door.
Mr. McVey (butcher) makes chicken for everyone from behind the meat counter, but most people are not ready to eat. There is a thud against the window and one of the men watching screams. There are bug-like creatures up to four feet long crawling all over the windows. One of the bags covering the window falls on a man and kills him. A giant dinosaur looking bird comes in through a jagged hole in the glass. It starts to rip the body apart. David sets it on fire with a torch. A bug gets in, but an older teacher lady, Mrs. Reppler, kills it with a few bottles of Raid. A girl named Amanda cleans it up. David tires to sleep, but doesn’t get much. He turns down what is left of sleep aids and finds himself having to distract himself from his attraction to Amanda.
Ollie takes David to show him two army boys that were hung. He theorizes on how they helped each other do it to themselves and how it has to do with the Arrowhead Project everyone has been talking about the past few months. They hide the bodies under bags of puppy chow so others won’t get scared. Amanda takes David to the manager’s office where they think about their significant others while they have sex. Miller wants to get out of the grocery store before Mrs. Carmody convinces people to make a human sacrifice (she has already gained 2 followers). He thinks the parking lot might be gone, but can see David’s 4x4 they could take to sports store down the street. David agrees to go on a trial run to the pharmacy next door.
Miller, Jim, Mrs. Reppler, Mike, Buddy, Ollie, and David prepare to go to the pharmacy next door. Mrs. Carmody gains another follower and seems to be energized by it while everyone else seems like they are aging far too quickly. The group of seven makes their way over to the pharmacy where they find a slaughter. David grabs two comic books for Billy before he sees something black on the floor that lets him know what killed the people there. He tells the others they have to get back to the grocery store. Suddenly there are spiders wrapping their webs to get them. They try to fight back but only David, Ollie, and Mrs. Reppler make it back.
David finds Ollie and tells him they need to get out of there. Mrs. Carmody has gained several more followers, including the butcher. They plan on who they will take and decide to leave early in the morning so they can sneak out. Ollie prepares a couple bags of groceries and leaves them by the register closest to the door. At quarter after 4am, he wakes up David. David wakes up the others they plan to take with them and explains their plan. He thinks they will be safe from the monsters as long as they get to his Scout before they catch their scent. Hattie, who has been watching Billy, thinks it is worth the risk for a chance to see the sun again. Amanda agrees. As they make their way to the front of the store, they find the doors blocked and Mrs. Carmody spilling their groceries. She convinces her followers they need to sacrifice Billy to appease God and receive salvation. As the mob approaches, Ollie shoots the old lady. He tells David he didn’t have any other choice and David thanks him. They get ready to leave.
The mad dash for the Scout doesn’t go as well as planned. Hattie is killed by one of the giant spiders. Ollie is cut in half by a lobster claw and dragged away, only able to get one shot at the thing. Carnell turns back to go inside again and makes it. Mrs. Reppler gets Amanda in the car and David gets Billy inside. David quickly reaches out for the gun and just barely gets it in. They drive past the store and David heads towards home. There are too many trees down for him to make it all the way and they start heading south. David drives slowly and just before the New Hampshire border, an impossibly large beast walks over them and all they can see are the legs ending in the clouds. They stop at a Howard Johnson’s and David flips through the radio stations. He thinks he hears a word before the static takes back over: Hartford. David finishes writing his story and leaves it on the counter.