In 1981, demonologists Ed and Lorraine Warren document the exorcism of 8-year-old David Glatzel, attended by his family, his sister Debbie, her boyfriend Arne Johnson, and Father Gordon in 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 while he suffers from a heart attack and is taken to a hospital in a comatose state.
The following month, 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 kennel where Debbie and Arne’s apartment is and where Debbie works, warning them that a tragedy will occur there. Arne and Debbie return to their apartment. After feeling unwell and seeing supernatural visions, Arne murders his landlord, Bruno Sauls, under the influence of demonic possession by stabbing him 22 times. 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 girlfriend 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 corpse’s hand to help find the location of the occultist. Lorraine, in a vision, travels through a dark tunnel and witnesses the occultist attempting to have Arne kill himself but stops her just in time. Lorraine is threatened by the occultist, and she tells Ed that the connection works both ways.