The morning after the small town of Bridgton, Maine is struck by a violent thunderstorm, commercial artist David Drayton, his son, Billy, and his wife, Stephanie, witness a mysterious mist moving across the lake towards their lakeshore home.
Concerned with cleaning up the aftermath of the storm, David, Billy and their neighbour, Brent Norton, head into town, to the local grocery store, where the patrons' attention is drawn by an unusual amount of police and military activity. A bloody-nosed Dan Miller rushes in, ranting about something in the mist taking John Lee.
The patrons watch as the mist moves across the parking lot towards the market and engulfs it. The market is then struck and damaged by violent tremors. A woman with kids at home eventually asks for someone to escort her home, but everyone refuses, so the woman goes out into the mist alone, towards her car.
When David searches the loading dock for something to cover Billy up with, he hears things slithering against the outside of the loading door. Mechanics Jim Grondin, Myron LaFleur, deputy manager Ollie Weeks and Norm the bag-boy attempt to fix the generator by sending Norm out to unblock the generator exhaust pipe, but as the loading door is opened, he is dragged to his death by a swarm of flesh-eating tentacles.
The others report the situation to the people in the market, but some, including Brent Norton, are sceptical, and mentally unstable religious fanatic Mrs. Carmody is convinced that the End of Days has come. Once the danger is confirmed to only be outside, the patrons who either believe or are in little denial barricade the front store windows with bags of soil and dog food and duct tape the cracks caused by the tremors.
Norton eventually gathers a small group of followers who are in denial of the dangers in the mist, and David and his followers fail to convince them not to go out into the mist. Before they leave, however, a biker who goes with Norton's group to protect them, ties a 300-foot clothsline around his waist to tell the remaining patrons when they get past that distance.
At approximately 200 feet, however, Norton's group is attacked by a large, unseen creature and David and the others are able to drag back only the biker's severed body, proving to the remaining patrons that there are dangerous creatures in the mist.
At night, Scorpion-Flies - plump, flying creatures - land on the store windows, and the large, pterodactyl-like Pterobuzzards follow them and pluck them off the store windows to eat, two of the Pterobuzzards eventually breaking into the store. Tom Smalley and Sally are both killed, but the Pterobuzzards and Scorpion-Flies which have infiltrated the store are killed and the broken window closed up.
The next morning, an expedition to the nextdoor pharmacy lead by David and consisting of Ollie, Jim, Irene Reppler, Jessup, Dan Miller, Bobby Eagleton and Mike Hatlen is made to bring back supplies for a horribly burnt Joe Eagleton. The group arrive at the pharmacy and find the supplies they need, but soon realize it is overrun with Gray Widowers; large, spider-like predators which produce corrosive webs. Bobby and Mike are all killed in the ensuing battle, and David, Ollie, Dan, Jessup, Jim and Reppler narrowly escape back to the market. Joe died from his wounds while they were were in the pharmacy, making the expedition fruitless.
As a result of the failed expedition, the remaining survivors in the store are divided into three groups: David's group, which consists of the still-sane, rational-minded survivors, Mrs. Carmody's congregation, which consists of a large number of survivors who have been driven insane by the trauma, and now follow Mrs. Carmody; the latter group now includes a shaken Jim and Mr. Mackey. Finally, the third group is a few people who side with neither David, nor Mrs. Carmody.
David's group eventually discover two soldiers from the military base have committed suicide via hanging, and demand from Jessup what the Arrowhead Project was, and how it caused the disaster.
Jim then appears, having noticed the group acting suspicious and following them, and drags Jessup out. Jessup reveals to Mrs. Carmody and her followers that the Arrowhead Project was an experiment to peer into other dimensions, and was, according to Donaldson (one of the soldiers), the origin of the mist. Mrs. Carmody's followers then sacrifice a terrified Jessup by throwing and locking him out of the market to be killed by the creatures. As Jessup pleads to be let back in, an Arachni-Lobster approaches and kills him.
The next morning, David and his group are about to use David's car to escape the store, but they are intercepted by Mrs. Carmody, who orders her followers to sacrifice first Billy, then Amanda, then David's entire group. However, before her followers are able to do so, Mrs. Carmody is shot and killed by Ollie, causing her horrified followers to snap back to reality, back down, and allow David's group to leave.
David, Billy, Amanda, Reppler and Dan all make it to the Land Cruiser, but Ollie is cut in half by the Arachni-Lobster, Bud retreats back to the market, and Myron and Ambrose are both killed by Gray Widowers.
David returns home to get Steff, but finds his home destroyed, and his wife dead, having fallen prey to the Gray Widowers. Heartbroken, David and the group drive south to try and get clear of the mist, but only find the destruction left in the wake of the mist.
They keep driving until they eventually run out of fuel, and as Billy wakes up from his nap, David shoots and kills him and the three other people in the car with the last four bullets in Amanda's gun to give them a quicker and more pleasant death. David then exits the car to be killed by the monsters in the mist, but instead sees a military convoy pass by and the mist dissipate.
David sees survivors in the convoy, including the woman with kids at home and people from the market. Horrifyingly realizing that he was driving away from rescue, that he needlessly killed his own son and three other people just moments before they would have been rescued, and that they would have been rescued anyway if they had just stayed in the market, David falls to his knees screaming as two soldiers watch in confusion.
- Thomas Jane as David Drayton
- Marcia Gay Harden as Mrs. Carmody
- Laurie Holden as Amanda Dumfries
- Andre Braughter as Brenton Norton
- Toby Jones as Ollie Weeks
- William Sadler as Jim Grondin
- Jeffrey DeMunn as Dan Miller
- Frances Sternhagen as Irene Reppler
- Nathan Gamble as Billy Drayton
- Alexa Davalos as Sally
- Chris Owen as Norm
- Sam Witwer as Private Wayne Jessup
- Robert C. Treveiler as Bud Brown
- David Jensen as Myron LaFleur
- Melissa McBride as Woman with kids at home
- Andy Stahl as Michael Hatlen
- Buck Taylor as Ambrose Cornell
- Brandon O'Dell as Bobby Eagleton
- Jackson Hurst as Joe Eagleton
- Brian Libby as Biker
- Susan Malerstein as Hattie Turman
- Mathew Greer as Silas
- Juan Gabriel Pareja as Morales
- Walter Fauntleroy as Donaldson
- Amin Joseph as MP
- Kelly Collins Lintz as Stephanie Drayton
- Ginnie Randall as Woman 1
- Tiffany Morgan as Woman 2
- Kim Wall as Terrified Woman
- Julio Cedillo as Father
- Ron Clinton Smith as Mr. Mackey
- Gregg Brazzel as Tom Smalley
- Sonny Franks as Man with El Camino
- Dodie Brown as Screaming Woman
- Taylor Brown as Screaming Child
- Jay Amor as Shopper 4
- Kristin Barnhart as Rescued Refugee
- Kevin Beard as Shopper 2
- Derek Cox-Berg as Teenager 1
- Brian Scott Hunt as Teenager 2
- Cherami Leigh as Teenage Girl
- Ted Ferguson as Norton's Follower
- Amy McGee as Mrs. Carmody - Non-believer
- Ritchie Montgomery as Paisley Hat Man
- Michaela Morgan as Little Girl
- Kip Cummings as Mrs. Carmody's Follower
- John F. Daniel as Mrs. Carmody's Follower
- Tammy Eaton as Shopper
- Ron Fagan as Shopper
- Travis Fontenot as Hazmat Worker
- Louis Herthum as Colonel
- Walt Hollis as Bio Hazard Engineer
- Pamela Houghton as Mrs. Carmody's Follower
- Mike Martindale as Sentry
- Cindy McBride as Mrs. Carmody's Follower
- Eric Kelly McFarland as Military Soldier
- Darrick Mosley as Shopper
- Chuck Vail as Military Soldier
- Steven E. Williams as Mash Group Member
- Unknown as Cashier 2
- Unknown as Teenage Girl
Differences from the novella
- The scene in which David warns Billy not to go near the fallen utility poles, and assures Steff that Billy knows, is omitted.
- In the novella, Norton's car, which was crushed under a fallen tree, was a 1960 Thunderbird, whereas in the film, it was a 1980 Mercedes.
- In the novella, David's car was an International Scout, but in the film, it was a 1968 Toyota Land Cruiser.
- In the novella, David parked his car in front of the store, but in the film, it was parked at the opposite end of the parking lot from the store.
- Mrs. Carmody is much younger in the film than in the novella.
- Private Jessup and the MP are completely omitted from the novella.
- The scene in which John Lee goes out from the market into the mist is omitted.
- In the novella, the bloody-nosed man who runs into the market, screaming about something in the mist taking John Lee is unknown, whereas in the film, it is Dan Miller.
- Dan appears to be older in the film than in the novella.
- In the film, Ollie helped David try to save Norm from the Tentacles from Planet X, whereas in the novella, he simply stood by, staring in shock at the Tentacles.
- In the novella, Norm's apron is ripped off by a Tentacle, while in the film, it mysteriously disappears while the Tentacles are trying to drag him out.
- David lashed out at Jim in the film instead of Myron.
- In the film, when the severed Tentacle is poked, it dies and turns to sizzling liquid.
- There is a scene in the film where Amanda attempted to befriend Mrs. Carmody, but was nastily rebuffed. This scene was not in the novella.
- Irene is shown to completely hate Mrs. Carmody in the movie and shows this by throwing a can of peas at her to shut her up, but in the novella, she has absolutely no interaction with Mrs. Carmody whatsoever.
- In the novella, the barricade of fertilizer bags placed over the store display windows went as high as the windows, whereas in the film, the barricade was only five feet high.
- In the novella, Mrs. Carmody was hit by an unnamed man, but in the film, it was Amanda Dumfries.
- In the film, Norton's follower known as "Golf-Cap" is replaced with the Biker.
- When Norton's group was attacked in the mist, the people at the market dragged the clothesline tied around Golf-Cap/the biker's waist back to the store; in the novella, it was severed and covered in blood, whereas in the film, the lower half of the biker's bisected body was still tied to the end.
- In the novella, only one Pterobuzzard and one Scorpion-Fly get into the store; the former is burnt to death right after killing Tom Smalley, while the latter is killed by Ms. Reppler with a can of Raid. But in the film, five Scorpion-Flies and two Pterobuzzards broke into the store. Sally was stung by a Scorpion-Fly and died; one Scorpion-Fly was hacked to death by Tom Smalley, another was crushed to death by Amanda, and the other three were eaten by the Pterobuzzards. The second Pterobuzzard was burnt and beaten to death by David with a torched mop after killing Tom Smalley, while the first flew around the store, eating Scorpion-Flies and causing panic and havoc, until it was shot in the wing by Ollie, disabling its flight. The Pterobuzzard was then about to attack Billy, when Ollie fatally shot it from behind.
- In an early storyboard for the film, after the Scorpion-Flies and Pterobuzzards break into the market, a giant, centipede-like creature crawls into the store through the hole in the window.
- Joe Eagleton is omitted from the novella.
- In the film, shortly after the Scorpion-Flies and Pterobuzzards breached the market and were killed, Hattie committed suicide via drug overdose.
- In the novella, Ollie merely speculated that the Arrowhead Project caused the disaster, whereas in the film, this was stated by Jessup to be very likely.
- In the novella, Amanda warned David that if the other people in the market were pushed back the breaking point, Mrs. Carmody could get them to turn to human sacrifice; but in the film, during a discussion between David's group about this, Amanda was the only one who insisted that people of a civilized society would never do that.
- The scene in which David and Amanda have an affair is omitted.
- In the novella, David's group went to the nextdoor pharmacy for supplies for Mrs. Clapham, who was injured the previous night, whereas in the film, it was to get supplies to treat Joe's burns.
- In the film, Jim and Dan both survived the expedition to the pharmacy.
- In the film, Jim joins Mrs. Carmody's congregation instead of Myron.
- In the film, the scene in which David discusses his theory that the creatures in the mist hunt by smell is omitted, leaving the reason the creatures in the mist (except for the Scorpion-Flies, who are attracted to light) only attack buildings with open doors or windows and people who are outside in the mist unexplained.
- In the novella, Mrs. Carmody was shot once in the stomach by Ollie, and warned David's group that they would die out there before she died. But in the film, Ollie shot Mrs. Carmody once in the abdomen, then finished her off with a shot to the head. In the movie script, In the movie script, Ollie intended to shoot her only one time and let her suffer a slow death, but as David and his group were passing her by, she managed to yell out an order to kill David's group one last time., forcing Ollie to completely kill her.
- In the film, Myron went out with the group into the mist, and was subsequently killed by a Gray Widower, while in the novella, he stayed in the market.
- In the novella, Bud refused David's offer to try and escape the store with them. However, in the film, he went out into the mist with them, but ran back to the store when he saw a Gray Widower kill Myron.
- In the novella, Ambrose ran back into the store when the group was attacked by the creatures, but in the film, he was attacked and killed by a pack of Gray Widowers in the parking lot.
- In the film, Dan escapes to David's Land Cruiser with David, Billy, Amanda and Reppler.
- In the novella, David was unable to drive back to his home to get Steff, leaving her fate ambiguous. But in the film, David was able to drive back to his home, and found that Steff had been killed by Gray Widowers.
- The scenes with the Killer Kite, the Green Fly and the Tentacles from Planet X blocking the road, are omitted.
- There was a scene that was not in the novella where, while they were driving south through the mist, David and his group discovered a school bus; it had been attacked by Gray Widowers which had killed and cocooned all the children on board.
- In the novella, only the Behemoth's legs were seen, whereas in the film, the creature's entire body was seen (although it was partially obscured by the mist).
- In the novella, David and his group kept driving through the mist towards Hartford, into an uncertain future. However, in the film they kept driving until the car ran out of fuel. David then used the last four bullets in Amanda's gun to kill the other four people in the car. Minutes later, a military convoy carrying survivors passed by, leaving David screaming on his knees at the realization that he killed his own son and three other people for nothing.
- In the novella, the fates of those left behind in the market were never known. In the film, people from the market, many of them Mrs. Carmody's ex-followers, are very, very briefly glimpsed abord the rescue trucks at the end, implying that everyone left in the market were rescued shortly after David and his group left.
- In the film's opening scene, the picture David is painting is of Roland the Gunslinger from Stephen King's The Dark Tower series.
- In the original script for the film, a prologue scene showed that the Arrowhead Project was indeed the cause of the mist, but the scene was scrapped to leave the origins of the mist as more ambiguous. In the scene, the military personnel at the Arrowhead Project base successfully opened a portal to another dimension in a containment tank. However, lightning from the storm struck the base, causing the portal to expand and allow the mist and the creatures in it through into the tank. Despite the personnel's attempts to close the portal, the creatures in the mist broke the tank open, causing the mist to pour into the base.
- If one looks closely, the books in the grocery store's book rack are all Stephen King novels.
- The pharmacy scene is similar to a scene from Aliens.
- The pharmacy next to the Food House store is called "King's Pharmacy", most likely a reference to author Stephen King. Coincidentally, Stephen King himself once had a cameo as a pharmacist in the film adaptation of his novel Thinner.
- When Norm is attacked by Tentacles from Planet X in the loading dock, his apron mysteriously disappears while the Tentacles are attempting to drag him out.
- Near the beginning and at the end of the film, the military were using jeeps. However, the US Army hasn't used jeeps since the 1980s.
- The MP's uniform dates back to the 1960s.
- Mrs. Carmody is apparently a devout Christian, however she referred to the book of "Revelation" as "Revelations".
- In the film, Mrs. Carmody is shown to swear several times. However, she is a highly religious Christian, and Christians consider swearing to be a sin.
- If one looks closely when Mrs. Carmody is staring out into the mist and proclaiming it to be death, in the reflection in the store windows, the mist has completely vanished.
- The blood spatter on the windshield of David's car goes from being in the middle of the broken glass to being spread out across the windshield and then back again.
- In the scene where Sally is stung by a Scorpion-Fly, the mark from the sting appears on her neck before the Scorpion-Fly strikes.
- When David is seen repeatedly beating down a Pterobuzzard in the grocery store with a mop, there is one brief camera shot that clearly shows the end of the mop still attached after it was removed moments earlier. The next shot shows the mop without the end.
- In the scene where Norm is being dragged by the Tentacles from Planet X under the loading door, initially, there is no visible blood trail behind him. Yet a short while later, when Ollie goes to stop the generator, and when David, Ollie, Bud and the Biker search the storeroom, there is a thick trail of blood left beside the axe from when the Tentacles dragged Norm out.
- Central Maine Power (CMP) trucks have an orange cargo area with a white cab, unlike those shown in the film.
- When David shoots Billy, Amanda, Irene and Dan, the shot of the Land Cruiser from the outside shows no blood spattered on the windows. However, in the next shot and all the following shots, blood is spattered across the windows.
- When Mrs. Carmody is shot in the head by Ollie, the bullet punctures through the back of her skull and sprays blood on the floor behind her, but the shot of her body shows no blood behind her except for the puddle pooling around her head.
- Right before Mrs. Carmody is shot, an elderly woman is standing right behind her and cheering her on, but the woman is gone in the very next shot.