Mia Medina, alongside cameraman Luis and sound technician Gavin, begins filming a thesis documentary about Alzheimer’s disease in rural Virginia. They focus on Deborah Logan, a former town switchboard operator whose daughter Sarah reluctantly agreed to the project to secure funds to pay off the mortgage on their secluded home. While Deborah initially shows classic symptoms of early-stage cognitive decline, her behavior rapidly deteriorates into erratic, uncharacteristic aggression.

Stationary cameras placed throughout the home record disturbing anomalies: Deborah peeling off her own skin until her fingers bleed, speaking fluent French—a language she never learned—and levitating atop a high kitchen counter in the middle of the night. A local neighbor and old family friend, Harris, insists that Deborah should be hospitalized, while medical doctors diagnose her with a rare, aggressive neurological variant. Gavin quits the documentary crew in terror after witnessing strange switchboard connections operating without electricity.

Investigating the history of the property and Deborah's switchboard records, Mia discovers a sinister connection to Henri Desjardins, an eccentric French-Canadian physician who murdered four young girls decades earlier in ritualistic attempts to gain occult immortality before mysteriously vanishing. Deborah’s house sits directly adjacent to Desjardins’ former property. When Deborah's medical tests reveal venomous snake toxins in her bloodstream and identical runic symbols burned into her skin, Mia and Sarah realize Deborah is being possessed by the parasitic spirit of Desjardins, who needs a fifth virgin victim to complete his interrupted ritual.

Deborah is admitted to a secure psychiatric wing, but Desjardins' spirit takes full physical control. She abducts a young girl named Cara, a fellow cancer patient from the pediatric ward, and flees into the dense woods near the estate. Armed with cameras and flashlights, Sarah, Mia, and a police deputy track Deborah into the subterranean mines beneath the forest where Desjardins performed his historical sacrifices.

In the depths of the underground caverns, Sarah and Mia discover Deborah in a catatonic, monstrous state. In a grotesque display of biological corruption, Deborah’s jaw disarticulates completely like a snake, attempting to swallow the screaming Cara’s head whole.

Sarah and Mia intervene, confronting the possessed woman. Realizing that Desjardins' soul is anchored to his unburied remains, Sarah locates the decayed, ritualized corpse of Desjardins preserved in the cave chamber and sets it ablaze with lighter fluid. As the skeleton burns, the demonic entity is expelled from Deborah’s body, causing her to collapse into human frailty, saving Cara's life.

An epilogue news broadcast reveals the aftermath: Deborah was judged unfit to stand trial for kidnapping due to severe cognitive degeneration and returned to a psychiatric hospice. Sarah continues caring for her mother, whose condition has stabilized into quiet dementia. The final shot cuts to Cara celebrating her tenth birthday with her cancer in full remission, turning to look directly at the camera with an unsettling, knowing smirk, hinting that Desjardins’ malevolent consciousness may have slipped into the young girl instead.