Kevin Delevan gets just what he wants for his fifteenth birthday — a Polaroid Sun 660 camera. Soon though, he notices that there is something strange about the camera. It prints similar pictures all the time but never what the camera is pointed at. Instead, it shows a street with a white fence and a black dog in the distance. Kevin takes the camera to Reginald "Pop" Merrill's junk shop, The Emporium Galorium, in hopes that Pop can fix it. When Pop sees the pictures, he asks Kevin to continue taking pictures with the camera on a regular basis. During this time, Kevin begins to see that the pictures are progressively showing movement, with the dog walking along in front of the fence. In the last set of pictures he takes for Pop, Kevin sees that the dog has stopped walking and is turning towards the camera. He also notices that the dog is wearing a string tie that belongs to him, which scares him into wanting to destroy the camera.
When Kevin's father John Delevan finds out that Kevin has been talking to Pop, he tries to intercede. Pop had once loaned John money at an outrageous interest rate, giving John an extreme distrust and dislike for the man. After Kevin and John turn the pictures over to Pop, Kevin tells him about his intentions to destroy the camera, and Pop fools them by switching Kevin's camera with a different one. When Kevin smashes the other camera, Pop begins to visit the "Mad Hatters," some of his customers who are collectors of paranormal items, with the intent of selling the camera. When each of these customers refuses to buy it, Pop comes to believe that Kevin was right, and he is better off destroying the camera. As Pop had been showing off the camera, he had taken more and more pictures, and in these the dog had turned fully toward the camera and started hunch down, as if preparing to pounce. When Pop returns to his junk shop, he attempts to destroy the camera, but in a state of hallucination, he instead destroys a clock, believing it to be the camera.
Meanwhile, Kevin has been having recurring nightmares about the other world in which the dog (which he now refers to as the "Sun Dog," from the name of his camera) exists. In these nightmares, he sees various people who offer him warnings, and eventually the dog begins to chase him, with Kevin waking at the last minute before the dog finally catches up to him. These nightmares lead Kevin to realize that Pop had tricked him, and when he informs his father, John also sees through Pop's ruse. The two head to The Emporium Galorium to confront Pop and finally destroy the camera.
Pop, still under the delusion that he had destroyed the camera, goes to the nearby drug store and buys film for the camera, hallucinating that he is buying pipe tobacco. Kevin and John show up shortly after Pop leaves, and Kevin, acting upon the ambiguous warnings from his dreams, buys a new Polaroid Sun camera. When Kevin and John get to Pop's junk shop, they see the flashes as Pop continues taking pictures with the "paranormal" Polaroid Sun. With each flash, the sound of the dog's growls become louder and louder, and by the time Kevin and his father reach the shop's door, the noise is so loud it shatters all the windows.
When they enter the shop, the camera in Pop's hands has began to melt. As Pop takes the very last picture, the camera explodes and molten plastic hits Pop's neck, burning him fatally. The last picture begins to warp and bubble, and inside the bubble, the Sun Dog begins to grow. Before the dog can break the bubble's membrane and attack, Kevin takes its picture with the new camera he bought at the drug store. The dog turns to stone, sinks back into the world behind the warped photo, and disappears.
The next year, Kevin receives a computer for his birthday. After Kevin types out the common test phrase "The quick brown fox jumps over the lazy dog," the printer prints out a different message:
- The dog is loose again.
It is not sleeping.
It is not lazy.
It's very hungry.
And it's VERY angry.
The audiobook version of "The Sun Dog" is read by Tim Sample.