Washed-up true-crime author Ellison Oswalt moves his wife Tracy, twelve-year-old son Trevor, and seven-year-old daughter Ashley into a new home in rural Pennsylvania. Desperate to replicate the success of his ten-year-old bestseller Blood Diner, Ellison secretly moves his family into the exact house where the Stevenson family was hanged from a tree in the backyard, whose young daughter Stephanie vanished without a trace. The local Sheriff warns Ellison that his presence is unwelcome, but his enthusiastic deputy—an avid reader—agrees to assist his research off the record.

While unpacking in the attic, Ellison discovers a cardboard box containing a Super 8 projector and several metal film reels labeled like benign home videos: "Pool Party '66," "BBQ '79," "Sleepy Time '98," and "Family Hanging Out '11." Setting up the projector in his office, Ellison watches the footage, horrified to discover that each reel depicts an entire family being brutally murdered by an unseen cameraman: a family drowned together on lawn chairs in a pool, burned alive inside a car trunk, having their throats slit in bed, and the hanging that occurred in his own backyard. In each reel, Ellison spots a demonic, pallid face watching from the background.

Ellison digitizes the footage on his computer, using photo enhancement software to isolate the figure and a strange runic symbol painted in blood at the scenes. Trevor begins suffering severe night terrors, sleepwalking and screaming inside laundry boxes, while Ashley paints an unsettling picture of missing girl Stephanie on her bedroom wall. Consulting Professor Jonas, an expert in occult mythology, Ellison learns the symbol belongs to Bughuul, a Babylonian deity known as the "Eater of Children." Jonas explains that Bughuul consumes the souls of human children, using recorded images as a psychic doorway to cross from his realm into the mortal world.

The disturbances escalate as the projector runs on its own at night, and ghostly apparitions of the slaughtered children silently walk through the house. When a package arrives with an "extended cut" reel revealing that the missing child in every family was the brainwashed murderer who recorded the footage, Ellison's sanity snaps. Confronted by Bughuul in the attic, Ellison burns the projector and films in a metal fire pit outside, wakes his family in a panic, and moves them immediately back to their previous house.

Back in their old home, Deputy So & So calls Ellison with a revelation derived from decades of police files: every murdered family previously lived in the home where the prior murder occurred, and the murders only occurred after the family moved away from the crime scene house into a new location. By moving his family out of the Stevenson house, Ellison has completed the ritual chain.

Ellison looks up into his attic and finds a pristine cardboard box containing the projector and the intact film reels, along with a newly labeled envelope: "Chop Talk '12."

Feeling suddenly dizzy, Ellison collapses onto the floor, realizing his coffee has been poisoned. As his vision blurs, his young daughter Ashley steps out of the shadows holding an axe, having painted the walls with his family's blood. Ashley tells her father that she will make him famous again, whispering that Bughuul promised she could live in his realm if she recorded her art.

Ashley films herself brutally dismembering Ellison, Tracy, and Trevor with the axe, painting Bughuul's bloody sigil and macabre murals of her victims across the walls using their blood. Ashley places the finished reel into the cardboard box in the attic. Bughuul manifests in the shadows, gently picks up Ashley in his arms, and walks into the glowing projection screen, disappearing into the celluloid void. The film cuts to the attic box sitting in darkness, before Bughuul lunges directly into the camera lens with a screeching roar.