The film opens with Super 8 footage depicting a family of four standing beneath a tree with sacks over their heads and nooses around their necks. An unseen figure saws through a branch acting as a counterweight, causing their deaths by hanging.
Months later, true crime writer Ellison Oswalt (Ethan Hawke) moves into the murdered family’s home with his wife, Tracy (Juliet Rylance), and their two children, 7-year-old Ashley (Clare Foley), an artist who is allowed to paint on her walls, and 12-year-old Trevor (Michael Hall D’Addario), who suffers from night terrors. The camera moves and the tree (with the sawn-off branch propped against it) comes into view, revealing to the audience that the house is the one where the family was murdered.
Only Ellison is aware that the house they are moving into was the crime scene from the opening scene. Ellison intends to use the case of the murdered family as the basis for his new book, and hopes that his research will reveal the fate of the Stevenson family’s fifth member, a 10-year-old girl named Stephanie who disappeared following the murders.
That night, during dinner, Ellison and his family talk about the book. While he is working, he hears noises, but it is revealed that it is just Ashley, unable to find the bathroom. Later that night Ellison discovers Trevor in a box, naked and screaming, having experienced another night terror. He carries him back inside.
Ellison finds a box (along with a black scorpion which terrifies him) in the attic that contains a projector and several reels of Super 8 mm footage that are each labeled as innocent home movies. Ellison discovers that the films are actually snuff films depicting different families being murdered in various ways by an unseen person holding the camera. The families are being tied to sunbeds and drowned in their pool (Pool Party ’66), being bound and gagged and burned to death in a car (BBQ ’79), being run over by a lawn mower (Lawn Work ’86), being bound and gagged and having their throats slit in bed during the night (Sleepy Time ’98), and the hanging from the opening of the film (Family Hanging Out ’11).
The drowning film is especially disturbing for Ellison when he notices the face of a demonic figure watching the drownings from the bottom of the pool. The figure turns to face the camera, and Ellison pauses the film. He stares at the image for some time, but before he can discover what it is, the film catches fire and burns away.
He repairs the remainder of film as best he can but is left with only a blurry profile of the figure’s face as the clearest views of it have been destroyed by the fire.
Ellison, after re-watching all the movies, eventually finds the same demonic figure from the drowning film throughout all the films, watching the murders, along with a strange painted symbol, illustrated on the hood of the car in “BBQ ’79”, and in various other places throughout the films (such as the wall of the upstairs level in “Sleepy Time ’98”). Upon inspecting the inside of the lid of the box containing the films, and a large cornsnake, Ellison discovers childish drawings depicting the murders, along with crude scribbles of the demonic figure, labelled “Mr. Boogie,” before falling through his attic floor. Ellison phones 911, but when the operator asks several times how she can help him, Ellison ends the call and broods over what he has just seen.
Consulting a local deputy (James Ransone), Ellison discovers that the murders depicted in the films took place at different times, beginning in the 1960s, and in different cities across the country. He also learns that some of the families were drugged before being killed, and that a child from each family went missing following every murder.
The deputy refers Ellison to a local professor, Professor Jonas (Vincent D’Onofrio), whose expertise is the occult and demonic phenomena, symbols and paraphernalia, to decipher the symbol in the films. Professor Jonas tells Ellison that the symbols are that of a Pagan Babylonian deity named Bughuul (Nick King), who would kill entire families and then take one of their children into his realm in order to consume his/her soul, leaving the symbol behind as a clue. As the deputy calls Ellison to give him information on the families murdered, a paused image of Bughuul turns to Ellison, but returns to its regular position before he can notice. After reviewing the footage he filmed during the time he was in the attic and as he fell through, he catches a strange frame. Reviewing the footage, he witnesses many ghostly children’s hands grabbing him whilst falling, before shutting the laptop screen out of doubt.
One night, Ellison hears noises from the garden and goes to the window with a flashlight and a found image of Bughuul that he was carrying before he heard the noise. Ellison holds up the image to the bushes and realises the shrubbery in the image is identical to the ones in his back garden. Putting the image down reveals Bughuul standing in his bushes and Ellison rushes outside, armed with a baseball bat. However he only finds Trevor, having experienced yet another night terror and carries him back inside, where Tracy rushes into the room to help, having heard the noises and wonders what is happening.
Ellison returns outside into the dark garden to retrieve his bat when he encounters a black Rottweiler. The missing children suddenly appear behind Ellison in a white glowing light, causing the dog to flee out of fear. The next night, Ellison hears a noise within the home and investigates, again armed with his bat, but the missing children (whose bodies are badly rotted) follow him throughout the house, but when he hears them, they run out of sight, causing Ellison to believe he is going mad. After Ellison checks in on Ashley and leaves, Stephanie is seen to be hiding next to Ashley’s bed, painting her family’s hanging with Bughuul watching.
The following night, Ellison hears the film projector running and goes up to the attic. There, he finds the missing children seated in the attic watching one of the films. Bughuul suddenly appears on camera before physically appearing before Ellison, causing him to fall off the ladder. The projector, box and film reels are thrown down the ladder, while the noises stop and the attic lights are turned off. Ellison takes the camera, projector and the films to the backyard and burns them with petrol. His recently awakened wife meets him outside and he tells her that they’re moving back to their old house immediately.
At his old home, Ellison receives a message from Professor Jonas, who sends him scans of historical images associated with Bughuul (including the symbol seen in the snuff movies; the images have been partially destroyed by the early Christians, who believed that images of Bughuul served as a gateway for the demon to come from the spiritual realm to the mortal world; the images include the symbol from the murders along with a scorpion, a snake, (matching what he found in the attic) and a black beast. According to Professor Jonas, people, especially children, who saw the images of Bughuul could be possessed and even abducted into the images.
Ellison discovers the projector and films in his attic, along with a new envelope of film labeled “extended cut endings”. During this time, the deputy tries calling several times, but Ellison declines his calls. The next time the deputy calls, while Ellison is assembling the films, he finally answers. The deputy informs him that he has discovered a link between the murders: every family had previously lived in the house where the last murder took place, and each new murder occurred shortly after the family moved from the crime scene into their new residence (Stevenson family had lived in the same house in St. Louis where the Miller family had their throats slit. Miller’s lived in Orange County house where the 1986 lawn massacre occurred. The Orange County family lived in Sacramento house where the Martinez family was burned to death in their own car, and their previous home was the location of the 1966 Oregon drownings). By moving, Ellison has placed himself and his family in line to be the next victims.
Ellison fixes the extra film to the original reels and watches the footage, and finds that it depicts the missing children coming onscreen following each murder, revealing themselves to be the killers, apparently under Bughuul’s possession, before suddenly disappearing. Before he can react, Ellison becomes light-headed, and upon inspection of his coffee cup, he notices a bright green liquid mixed with the coffee and finds a note reading “Good Night, Daddy” under the cup before losing consciousness. Ashley appears behind Ellison, revealing herself to be the drugger, under Bughuul’s possession. Ellison awakens to find himself, Tracy and Trevor bound and gagged on the floor of the parlour in their old house next to the lit fire.
Ashley approaches holding the 8 mm camera, and promises him that she will make him famous again. Ashley then decapitates Tracy with an axe before going on to kill Ellison and Trevor, using their blood to paint images of cats, dogs and unicorns on the walls. With her work complete, Ashley views the Super-8 film of her murders while drawing the murder (matching the earlier drawings.) The film concludes with an image of the missing children staring into the camera. Bughuul appears, causing the children to flee in fear. He then lifts Ashley into his arms and disappears into the film with her.
The film concludes with an image of the box of films in the Oswalt family’s attic, now accompanied by Ashley’s reel, labeled “House Painting ’12”. The camera slowly pans away from the box, until Bughuul appears onscreen and the screen cuts out.