In 1976, paranormal investigators Ed and Lorraine Warren document the Amityville murders at the Amityville house, to determine if a demonic presence was truly responsible for Ronald DeFeo Jr. mass murdering his family on November 13, 1974 and the subsequent haunting incident involving the Lutz family. During a seance, Lorraine is drawn into a vision where she relives the murders and discovers a demonic nun figure, which strangles her. After seeing Ed being fatally impaled, Lorraine is able to break out of the vision.
One year later, in 1977, the Hodgson family begins to discover strange occurrences within their home in London. Janet, the second oldest of four children, is seen sleepwalking and conversing in her dreams with an entity in the form of an angry elderly man, 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 an elderly man, Bill Wilkins, who is revealed to have previously lived and died in the house. During the possession, Wilkins states that he enjoys tormenting the family and wants to claim back his home. 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 and prove whether or not it is a hoax. 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. She has yet another vision of the demonic nun in Ed’s study wherein the demon says its name, which Lorraine scribbles in her Bible.