A chilling tale unfolds with a dark ritual and a young boy, David, experiencing terrifying demonic possession. Paranormal investigators Ed and Lorraine Warren are called to assist the Glatzel family in their deepest despair. During a harrowing exorcism where the Warrens, a Catholic priest, and the family are present, the malevolent spirit latches itself onto Arne, David's sister's boyfriend. This transference goes unnoticed at the time, laying the groundwork for a story of true supernatural horror.

The aftermath of the exorcism sees Arne and Debbie trying to resume normal life, but the demonic entity that transferred to Arne begins to influence him, leading to a shocking and deadly confrontation. This incident propels the Warrens into an investigation intertwined with law enforcement and legal drama, as Arne claims demonic possession as a defense for his actions. The couple is drawn into a labyrinth of the occult as they uncover a curse at the root, and they work against time to prove the existence of the supernatural forces at play in an attempt to save Arne from his ominous fate.