The film begins with chilling Super 8 footage showcasing a family of four standing under a tree, hoods over their heads and nooses around their necks. An unseen figure mercilessly saws through a limb, leading to their grim hanging.
Months later, Ellison Oswalt, portrayed by James Ransone, a washed-up true crime writer, relocates to the home of the murdered family with his wife, Tracy played by Juliet Rylance, and their two children, Ashley—a talented young artist permitted to paint on the walls—and Trevor, who starts experiencing unsettling night terrors once they settle into their new abode. Unbeknownst to his family, Ellison is fully aware that their new home is the very site of a horrific crime, as he intends to base his upcoming book on the gruesome murders, delving into the fate of the family's missing fifth member, little Stephanie.
While rummaging through the attic, Ellison uncovers a box filled with a projector and several reels of Standard 8 mm footage, initially labeled as home movies. However, as he watches the films, he is horrified to find that they are recordings of sadistic murders, showcasing families meeting their gruesome ends in various ways—ranging from having their throats sliced (in "Sleepy Time '98") to burning alive in a car (in "BBQ '79"), drowning in their pool (in "Pool Party '66"), and being run over by a lawn mower (in "Lawn Work '86"). The opening sequence of hanging, labeled "Family Hanging Out '11," sets the gruesome tone for the chilling discovery.
The film depicting the drownings notably distresses Ellison as he spots an ominous demonic figure lurking at the bottom of the pool, its gaze piercing through the lens. Upon further inspection, he encounters this figure in every film and notices a strange symbol recurring throughout. Curiously, he finds crude drawings on the underside of the box lid, illustrating the murders and the menacing figure labeled as "Mr. Boogie."
In his quest for answers, Ellison consults a local deputy, who reveals that the harrowing events displayed in the films span several decades, beginning in the 1960s, and occurred across various cities. He learns that all the families were sedated before their slayings, and chillingly, a child from each family vanished following the murders. The deputy directs Ellison to seek insight from a local scholar, Professor Jonas (played by Vincent D'Onofrio), who specializes in the occult and demonic phenomena. Jonas elucidates that the ominous symbols represent Bughuul, a pagan deity notorious for devouring children’s souls after annihilating their families.
One fateful night, Ellison is drawn to the attic by the sound of the projector whirring to life. To his horror, he stumbles upon the rotting remains of the missing children, fixated on watching the macabre films. In a terrifying twist, Bughuul reveals himself not just on-screen but physically before Ellison—an encounter so chilling that Ellison decides to destroy the projector and its contents by burning them in the backyard, hoping to sever the curse. He then awakens his family to share the news that they are returning to their former home.
Upon reaching their old residence, Ellison receives a crucial message from Professor Jonas, accompanied by scans of historical descriptions linked to Bughuul. The professor explains why numerous cultures damaged the symbols; they feared that Bughuul resided within such images, acting as gateways to his sinister realm. While rummaging through his attic, Ellison bizarrely finds the projector and films again, this time containing an envelope marked "extended endings." During his experimentation with the films, he receives multiple calls from the deputy, who has uncovered a harrowing connection between the murders—the chilling fact that each family had previously resided in the house where the last murder occurred, suggesting that by returning to his old home, Ellison has unwittingly endangered his family by placing them in the path of danger.
In his investigation of the footage, he discovers that it reveals the missing children revealing themselves as the culprits, only to vanish abruptly. Before Ellison can process the implications, he finds himself dizzy and stumbling across disturbing clues. Inspecting his coffee cup, he discovers a taunting note that reads, "Good Night Daddy," alongside a strange green liquid before succumbing to unconsciousness.
When he regains consciousness, Ellison finds himself, Tracy, and Trevor bound and gagged. Strangely, Ashley approaches, gripping the 8 mm camera and chillingly declaring her intent to restore her father's fame. An unsettling transformation unfolds as Ashley resorts to murder, wielding an axe to eliminate her family, painting grotesque images of cats, dogs, and unicorns with their blood on the walls. As her masterpiece completes, she adds an eerie drawing of "Mr. Boogie," locking in her ties to the wicked influence. She then watches the film depicting her gruesome acts, culminating in a horrifying sight of the missing children viewing her latest creation. Bughuul emerges, his hands stained with green blood, sparking chaos as the children flee. Ultimately, he lifts Ashley into his arms, pulling her into his malevolent world.
The film closes ominously, showing the box of films in the Oswalt family's attic, now accompanied by a reel titled "House Painting '12." Bughuul ominously appears, delivering a haunting message that the viewers themselves are the next targets.
