Set in the fictional small town of Castle Rock, Maine, the story follows 15-year-old Kevin Delevan, who receives a Polaroid Sun 660 camera for his birthday. When he takes his first photograph of his family, the developed image does not show them. Instead, it depicts a large black dog on a sidewalk before a dilapidated white picket fence. Kevin's 11-year-old sister, Meg, accidentally knocks the camera off the table, chipping its housing. That evening, Kevin and his father, John, examine nearly a dozen more photographs, all showing the same outdoor scene despite having been taken indoors. Mr. Delevan wants to exchange the camera, but Meg declares it "a Manifestation," something supernatural. Kevin quietly agrees and decides to keep it, feeling an inexplicable sense of both ownership and dread.
On the advice of his teacher, Mr. Baker, Kevin brings the camera to Pop Merrill, the shrewd, elderly proprietor of the Emporium Galorium, a cluttered junk shop in town. Pop cannot repair the sealed modern camera, but he notices something crucial: The photographs are not identical. Each is slightly different, suggesting movement. He asks Kevin to take 30 more pictures on a timed schedule. Before Kevin leaves, he notices that a shadow they dismissed as a tree is actually the silhouette of a person, the unseen photographer on the other side of the image.
Kevin returns with 58 numbered photographs and tells Pop he wants to destroy the camera. The later photos have revealed something that terrifies him. Pop persuades him to wait, hinting that he knows Kevin's father and once did him a favor. After Kevin leaves, Pop drives to Lewiston to have the photographs transferred to videotape and purchases a brand-new Polaroid Sun 660. He damages the new camera's housing to match Kevin's and plans to stage a fake accident.
On the walk to the shop, Mr. Delevan reveals his history with Pop. Years ago, as a struggling newlywed, he made a reckless $400 bet on a basketball game, money he did not have. When he lost, Pop lent him the money at 10 percent interest compounded weekly, and Mr. Delevan secretly took a brutal second job at a paper mill for months to repay the debt. He tells Kevin he does not hate Pop but does not trust him.
At the Emporium Galorium, Pop plays the videotape, each photograph displayed for one second. The crude film reveals the dog walking along the fence, pausing, and turning its head toward the camera to reveal a single malevolent eye and the beginning of a snarl. Kevin perceives the dog as three-dimensional in an otherwise flat Polaroid world, something monstrous rather than merely canine. The implication is plain: The dog intends to attack whoever stands behind the camera.
While the Delevans rewatch the tape upstairs, Pop goes downstairs, locks Kevin's real camera in a hidden drawer, and knocks the duplicate off the worktable with a shout. Kevin and his father find the substitute camera with its lens shattered. Using Pop's magnifying glass, Mr. Delevan identifies the object around the dog's neck: a string tie with a woodpecker clasp, identical to the gift Kevin's great-aunt Hilda sends him every birthday. Kevin, relieved, smashes the fake camera on Pop's chopping block, and they burn all 58 photographs. Back home, Kevin finds that one of his 13 string ties is missing: the one with the woodpecker clasp. Despite this unsettling confirmation of a supernatural link, both father and son feel the crisis has passed.
Pop, meanwhile, regards the camera as a potential goldmine and spends two weeks trying to sell it to wealthy collectors of paranormal objects. All three prospects he contacts reject it. Throughout these attempts, Pop notices the dog is growing more monstrous and less recognizably canine. He impulsively snaps photos and finds the creature closer each time, coiled to spring, smoke issuing from its nostrils.
The camera begins exerting a supernatural hold on Pop. He wakes at three in the morning having taken a photograph in his sleep, the image showing the dog-thing beginning its spring. He resolves to destroy the camera, but the next morning a compulsion overtakes him. He smashes an imitation cuckoo clock on his chopping block, believing it is the camera. Still in a trance, he buys Polaroid film at LaVerdiere's drugstore, frightening the clerk, Molly Durham. He returns, hangs the real camera on the hook where the clock had been, loads it with fresh film, and sits down to "repair" the clock, unaware of what he has done.
Kevin, meanwhile, has recurring nightmares about Pop photographing the dog. Waking suddenly, he remembers that when Pop handed him the camera to smash, the exposure counter read three, meaning it contained film. Kevin had used every exposure; the counter should have read zero. He realizes Pop substituted a different camera. Kevin stops his father and explains, and Mr. Delevan recalls how eagerly Pop volunteered to go downstairs alone. They rush to the Emporium Galorium, find it closed, and spot a Polaroid hanging from a hook inside. Around back, the smashed cuckoo clock confirms that Pop destroyed the wrong thing.
After leaving the shop, Kevin insists they buy a new camera before returning. His father protests, but Kevin says, "It's coming, Dad. Please. It's my life." At LaVerdiere's, Kevin grabs a new Sun 660. As they hurry back, brilliant white flashes erupt from the Emporium Galorium, accompanied by a deep growl.
Inside, Pop fires the camera again and again, still believing he is tinkering with a clock. The camera melts in his hands, and he cannot release it. The final photograph bulges outward, swelling into a three-dimensional bubble. The membrane splits and the Sun dog forces its way through: first its massive jaw, then its forelegs clawing at the desk. A chunk of dissolving camera ruptures Pop's jugular vein, killing him.
Kevin bursts through the shop door and loads the new camera. The Sun dog forces its upper body through the membrane between worlds. Kevin notices the object around its neck has changed from the string-tie clasp to Pop's pipe-cleaning spoon, suggesting the creature's target shifts to whoever possesses the camera. As the dog lunges free with fire licking from its jaws, Kevin presses the shutter. The enormous flash freezes the Sun dog into a black-and-white negative image, and the creature slides back through the hole, which collapses and vanishes. The camera shatters. Kevin drops it and weeps as his father holds him. They walk out past burning magazines into the daylight, Kevin telling his father, "That's how I had to shoot it, Dad," with a camera rather than a weapon.
In an epilogue set on Kevin's 16th birthday, Aunt Hilda has died and left the family nearly $70,000. Kevin receives a computer. That afternoon, he types a test sentence, but the printer outputs a warning: "The dog is loose again. It is not sleeping. It is not lazy. It's coming for you, Kevin." The message adds that the dog is "VERY hungry" and "VERY angry." Mr. Delevan, reading the screen, reflects that the original debt was what hurt you, but the interest was what broke your back. The Sun dog, it seems, is far from finished.