The film begins with paranormal investigators Ed and Lorraine Warren documenting the Amityville murders at the Amityville house in 1976, to determine if a demonic presence was truly responsible for Ronald DeFeo Jr.
Mass murdering his family on November 13, 1974. During a seance, Lorraine is drawn into a vision where she relives the murders and discovers a demonic nun figure, before seeing Ed being fatally impaled. After a struggle, Ed is able to break her out of the vision.
One year later, in 1977, the Hodgson family begins to discover strange occurrences within their home in London, England. Janet, the second oldest of four children, is seen sleepwalking and conversing in her dreams with an entity who insists that the house is his.
Eventually, all siblings of the house and their mother Peggy witness paranormal events occurring right before their eyes, forcing them to seek refuge with their neighbors. When the media attempts to interview the Hodgsons, Janet is possessed by the spirit of Bill Wilkins, an older man who previously lived and died in the house, and who wants to claim his territory. As Janet begins to show more signs of demonic possession, the story eventually reaches the Warrens, who are requested to assist the local Church in the investigation. Lorraine, in fear of her vision of Ed’s death becoming reality, warns him not to get too involved in the case, and reluctantly agrees to travel to London.
While staying at the Hodgson residence, Ed and Lorraine consult with other paranormal investigators, including Maurice Grosse and Anita Gregory, on the legitimacy of the case. They also attempt to communicate with Wilkins’ spirit, hoping to talk him out of harassing the family. One night, after the Hodgsons witness Janet being possessed, Gregory presents video evidence of Janet purposely wrecking the kitchen as if for a prank. Ed and Lorraine are then convinced to leave the family on their own, but soon they discover that the spirit of Wilkins is only a pawn, being manipulated to haunt Janet, while the true mastermind is the demonic spirit that has been haunting Lorraine in her visions.