Father Lancaster Merrin, an aged Catholic priest portrayed by Max von Sydow, embarks on an archaeological expedition in Iraq, where he is confronted by a series of eerie omens that suggest an impending calamity. His journey leads him to uncover a statue representing a demonic figure known as Pazuzu, signaling the dark forces at play.
Meanwhile, in the affluent neighborhood of Georgetown, successful actress Chris MacNeil, played by Ellen Burstyn, begins to experience ominous occurrences in her home. She lives with her twelve-year-old daughter, Regan, portrayed by Linda Blair, as well as her personal assistant Sharon and two housekeepers. Regan’s father is absent for undisclosed reasons. The strange noises coming from the attic, initially dismissed by Chris as rat-related, soon escalate as Regan's behavior shifts dramatically—displaying signs of anxiety and depression. One night, she climbs into Chris's bed, terrified that her own bed had started "shaking."
As Chris juggles her acting responsibilities, she encounters Father Damien Karras, a conflicted priest with a background in psychology, during the filming of her movie. As Karras grapples with his diminishing faith and struggles with the care of his elderly mother in a decrepit New York hospital, signs of unease multiply in Chris's household. Regan reveals that she has been using a Ouija board, claiming to connect with a spiritual presence, while cryptic acts of desecration occur at a nearby church.
As Regan's symptoms worsen, she undergoes a series of medical evaluations, but the doctors find no clear answers. During this time, Chris hosts a glamorous party, where Regan shocks guests with her erratic behavior, including an alarming prediction concerning an astronaut's fate. In the aftermath, Chris discovers violent occurrences in Regan’s room, leading her to seek psychological help. A tragic twist strikes when Burke, a friend and associate, is found dead outside the house under mysterious circumstances, deepening Chris's desperation.
In a quest for answers, Chris learns about the concept of demonic possession, though she initially meets the idea with skepticism. She is urged to recount her daughter's troubling behaviors to Father Karras, who finds himself reluctantly drawn into the situation. An encounter with Regan reveals her transformation; she speaks in tongues, showcases unnatural strength, and teeters on the brink of something profoundly sinister. Faced with evidence that points to possession, Karras is spurred into action.
Father Merrin ultimately returns to confront the demon within Regan, and a harrowing exorcism unfolds. As the battle between good and evil intensifies, Regan's body becomes a vessel for unimaginable horror, marked by attacks on both priests, prompting Karras to confront his own demons. In a desperate act of self-sacrifice, Karras ultimately offers himself to save Regan, sealing his fate, while she, freed from torment, remains blissfully unaware of the malevolence that once engulfed her.
In the aftermath, Chris and Regan prepare to depart their haunted residence, receiving a visit from Father Dyer. In a bittersweet moment, Chris reveals to Father Dyer that Regan has no recollection of her traumatic experiences, and as they bid farewell, Regan's innocent demeanor provides a glimmer of hope that life can resume, despite the darkness that lingered over their lives.
