In 1981, demonologists Ed and Lorraine Warren document the exorcism of 8-year-old David Glatzel, attended by his family, his sister Debbie, and her boyfriend Arne Johnson, and Father Gordon in the town of Brookfield, Connecticut. During the exorcism, Arne invites the demon to enter his body instead of David’s. Ed witnesses the demon transport itself from David’s body to Arne’s whilst he suffers from a heart attack, and is taken to hospital in an unconscious state.
Ed wakes up at the hospital and reveals to Lorraine that he witnessed the demon enter Arne’s body. She sends the police to the Glatzel household and warns them that a tragedy will occur there. Arne and Debbie return to their apartment located above a kennel where Debbie works. After feeling unwell, Arne murders his landlord, Bruno Sauls, by stabbing him 22 times under the influence of demonic possession. With the support of the Warrens, his case becomes the first American murder trial to claim demonic possession as a defense, resulting in the beginning of an investigation into David’s original possession. The Warrens later discover a satanic curse passed on through a witch’s totem, and meet with Kastner, a former priest who previously dealt with the Disciples of the Ram cult. He tells them that an occultist had intentionally left the totem, resulting in the creation of a curse on the Glatzels, causing the possession of David.
The Warrens travel to Danvers, Massachusetts, to investigate the death of Katie Lincoln, who was also stabbed 22 times. Detectives had found a totem at the home of Katie’s friend Jessica, who is missing. Lorraine initiates a vision to recreate the murder and discovers that Jessica had stabbed Katie under the influence of demonic possession, before jumping to her death off of a cliff, which allows detectives to recover her body. The Warrens travel to the funeral home where her body rests and Lorraine touches the hand of the corpse to help find the location of the occultist. Lorraine witnesses the occultist attempting to have Arne kill himself but stops her just in time. Lorraine is threatened by the occultist and tells Ed that the connection works both ways.