Plot

When a mysterious entity possesses a young girl, her mother seeks the help of two Catholic priests to save her life.

A visiting actress in Washington, D.C., notices dramatic and dangerous changes in the behavior and physical makeup of her 12-year-old daughter. Meanwhile, a young priest at nearby Georgetown University begins to doubt his faith while dealing with his mother's terminal sickness. A frail, elderly priest recognizes the necessity for a show-down with an old demonic enemy.—Andrew Harmon <aharmon@erols.com>

Georgetown, Washington, D.C. Regan, the once happy 12-year-old daughter of successful actress Chris MacNeil, shows erratic behavior. And soon, things get out of hand. As a result, Regan becomes violent, forcing the desperate mother to seek spiritual help. But there are forces beyond our comprehension. As weary Father Merrin and his skeptical assistant perform an intricate purification ritual to rid the girl of the disturbing symptoms, sheer supernatural terror threatens their lives and souls. Can the exorcist confront pure darkness and evil incarnate?—Nick Riganas

Accompanied by her 12-year-old daughter Regan, actress Chris MacNeil relocates to Washington, D.C., where she is filming a movie. Mother and daughter have a good relationship, but Regan begins to act strangely after a while. She undergoes various neurological tests, but doctors can find nothing to explain her behavior. As Regan's situation reaches crisis proportions, Chris turns to Father Karras, a Roman Catholic priest and psychiatrist, to see if an exorcism might solve their problem. Karras is incredulous, but the church eventually agrees, calling in Father Merrin, who had previously conducted an exorcism and came face to face with the demon.—garykmcd

A movie actress taking up temporary residence in Washington D.C. has her troubles. The script for the movie she's filming seems inadequate. Her ex, also the father of her adolescent daughter, Regan, neglects to call the girl on her birthday. And the attic has rats. Meanwhile, Father Karras, a priest and a psychiatrist, is losing his faith, and he's dealing with a sick mother who needs medical care he hasn't the money to provide. Another priest, the old and ailing Father Merrin, has just returned from Iraq with forebodings of evil. These three persons meet when the sweet and cheerful Regan turns foul-mouthed and violent. But her sickness is beyond the reach of a medical doctor or a psychiatrist. What Regan needs is an exorcist.—J. Spurlin

Father Lancaster Merrin (Max Von Sydow) is an elderly Catholic priest on an archaeological dig in Iraq. Merrin has a number of strange omens, including a near miss with a runaway horse drawn carriage, and a clock that stops ticking in mid-stroke. Finally, Merrin discovers a statue of a bizarre demonic figure.

Back in the United States, in Washington D.C.'s upscale Georgetown neighborhood, a successful actress named Chris McNeil (Ellen Burstyn) begins experiencing strange phenomena. Chris lives with her twelve-year-old daughter Regan (Linda Blair). Regan slowly begins to exhibit strange behavior, undergoing behavioral changes much like depression and anxiety. She turns up in Chris's bed one night, complaining that her own bed was "shaking".

Chris is working on a new movie in Georgetown with director Burke Dennings (Jack MacGowran). While filming a scene one day, she notices a young Catholic priest watching her; his name is Damien Karras (Jason Miller). Father Karras has a background in psychology and counsels' parishioners at a nearby church. The strange occurrences in the McNeil house begin to increase. Regan reveals that she has been playing with a Ouija board and claims that she has the ability to communicate with a spiritual entity all by herself.

Chris has an elaborate party at her home with a number of affluent guests. One of her guests is another Jesuit named Father Dyer (Rev. William O'Malley). She finds that Karras and Dyer are good friends. During the party, Regan appears dressed in her nightgown and urinates on the carpet in front of the guests while making an ominous statement to a prominent astronaut ("You're going to die up there"). After the guests leave, Chris bathes Regan and puts her to bed, but is startled by a loud sound from Regan's bedroom. She rushes back down the hall and discovers Regan's bed shaking violently, rising up off the floor with Regan on it. Chris jumps on the bed and it still levitates.

Chris subjects Regan to a series of medical tests to discover what the problem is. The doctors are unable to discover anything. They find her rising and falling up and down on the bed in a way that seems impossible for a human being. When they try to sedate her, she hurls them across the room with abnormal strength, speaking to them in what seems to be a male voice: "Keep away! The sow is mine!" Eventually they sedate her.

Out of options, they advise Chris to search for a psychiatrist, but they also reluctantly discuss another possibility: they mention the phenomenon of demonic possession and the rite of exorcism. Chris hears a deep male voice bellowing at Regan to "do it", and Regan screaming in protest. In the bedroom, Chris finds Regan plunging a crucifix violently into her vagina. As Chris watches in horror, her daughter's head turns completely around backwards, and she speaks to Chris, saying to her "Do you know what she did? Your daughter??".

Desperate, Chris arranges to meet with Father Karras, and when she mentions the notion of exorcism, Karras is almost amused. He tells her that exorcism is nearly unheard of, and that he doesn't know anybody who has ever performed one. Chris is distraught and convinces him to meet with Regan anyway. Karras is shocked by the girl's appearance; she is tied to the headboard of her bed, her face misshapen and covered in lesions, her voice deep and gravelly. Regan announces that she's the devil, and toys with Karras in a number of ways, seeming to make a drawer next to the bed open all by itself, then speaking to Karras in a number of languages. Karras remains unconvinced, and when Regan claims "Your mother's in here with us", (Karras's mother has recently passed away) Karras asks her what his mother's maiden name is. Unable to answer, Regan vomits spectacularly all over him. Karras is still not convinced that Regan is possessed. Chris pleads with him to help her obtain an exorcism, swearing that the "thing" in the bed upstairs is not her daughter.

Karras visits Regan again and records their conversation, during which he sprinkles Regan with water. He tells her it is holy water, and she begins to writhe in pain, seemingly going into a trance and speaking in a strange language. Later he tells Chris that it will be difficult to make a case with the Bishop for possession; the water he sprinkled on Regan was simply tap water, and was not blessed. The Bishop, and Karras himself, would consider Regan to be mentally ill and not possessed. Later, Karras uses his tape recordings of Regan's seemingly incomprehensible babble to discover that she is really speaking backwards, in English. A phone call from Sharon interrupts him; she summons him to the house to see Regan, not wanting Chris to see that's happening as they look at Regan's unconscious body, the words "help me" begin to materialize on her stomach, rising up in her skin like scar tissue.

Karras reluctantly agrees to try and get an exorcism for Regan, although he seems to have more in common with the doctors who recommended it as a form of shock therapy. The church calls in Father Merrin to perform the exorcism, with Karras assisting. Merrin has performed exorcisms in the past, including a difficult one that "nearly killed him", according to the Bishop. When Merrin arrives at the McNeil house, Regan bellows his name from upstairs, as if she knows him, and she makes strange animal sounds. He warns Karras about conversing with the demon and reminds him that the demon will mix lies with the truth to confuse and attack them.

When they enter Regan's bedroom, she immediately begins with a string of obscenities. Merrin and Karras recite the ritual of exorcism and Regan manifests strange phenomena such as levitation, telekinesis, an abnormally long tongue, and strange vomiting. She constantly curses the priests and emits evil laughter and verbal abuse. Regan begins to talk to Karras in the voice of his mother, and he starts to break down. Merrin sends him away; when he returns, he finds Merrin dead on the floor, the victim of a heart attack. Regan cackles gleefully, infuriating Karras, who grabs her and shouts at the demon, "Come into me! Take me!" The transference works almost immediately; Karras begins to transform, and Regan returns to her normal self. Before Karras can harm her, his "normal" personality breaks through for a split second and he commits suicide, hurling himself out Regan's window. He tumbles down the stairs outside Regan's window and lays dying in the street below. By chance, Father Dyer happens upon the scene and administers the last rites to his friend.

In a brief epilogue, we see Chris and Regan as they prepare to leave the house in Georgetown. They are visited by Father Dyer. Chris speaks with him privately and tells him that Regan doesn't remember anything about the possession or the exorcism. Regan then appears and greets him cheerfully, transfixed by Father Dyer's white collar. Before they leave, she suddenly hugs Father Dyer and kisses him. As Chris pulls away in their car, she orders the driver to stop for a moment and gives Father Dyer the religious medallion that belonged to Father Karras; in their struggle, Regan had torn it from his body, and it was in her bedroom all along.