In the Georgetown neighborhood of Washington D.C., actress Chris MacNeil starts to notice disturbing changes in the behavior and physical state of her 12-year-old daughter, Regan. Initially considered as psychiatric in nature, Regan's symptoms rapidly escalate to include violent outbursts and supernatural phenomena occurring around her. Desperate, Chris seeks medical help but the doctors fail to provide a reasonable explanation for Regan's condition. The situation worsens as Regan undergoes a series of inconclusive tests, and her behavior becomes increasingly erratic and dangerous, culminating in a particularly horrific incident at a party hosted by Chris.

Faced with her daughter's inexplicable condition, Chris turns to Father Damian Karras, a local priest and psychiatrist who is struggling with his own faith after the death of his mother. Karras is skeptical at first, but after witnessing the terrifying events firsthand, he comes to believe that Regan may be possessed by an evil entity. He seeks the aid of Father Merrin, an experienced exorcist, to confront the demonic presence. The two priests must prepare themselves for a grueling battle against an ancient and malevolent force that has taken hold of Regan, pushing the limits of their faith and endurance.