The film opens with at an archaeological dig in the ancient ruins of Hatra in the Iraqi desert. Here aged Catholic priest and veteran exorcist, Fr. Lankester Merrin, is given a stone talisman carved in the image of Pazuzu, an antediluvian demonic entity that used to be worshiped by the Babylonians, the original inhabitants of Iraq. The old priest’s face looks troubled, as he is familiar with the history surrounding the worship of the demon.
Then, the film cuts to Georgetown, where actress Chris MacNeil is on site shooting a movie about student protests. Her close friend and associate, Burke Dennings, directs the film. Chris lives on location with her daughter Regan. Regan is a sweet 12-year-old child who, like many children traveling with their parents, looks for ways to keep amused while her mother is away at work. Tragically, Regan has began to entertain herself using a Ouija board and has begun contacting, or rather has been contacted by what her mother and her friends assume to be her imaginary friend whom she refers to as “Captain Howdy.”
Not too long after her initial contact with the Captain Regan begins to act strangely, such as stealing and cussing, these acts are then followed by supernatural occurrences such as mysterious noises and Regan exhibiting unusual strength. These odd occurrences come to a head when Regan’s mother hosts a party at their house. Regan suddenly comes downstairs from her room, barefoot and clad in her nightgown, and walks up to one of Chris’ guests, an astronaut, to give him a chilling but cryptic warning or threat: “You’re gonna die up there.” After making the announcement she promptly urinates on the floor. The supernatural manifestations continue to worsen and become more overt, frightening Chris sufficiently to seek out professional help. She consults a number of medical professionals but all of them give the same diagnosis: Regan, despite what she is showing, is physically healthy. The doctors instead sedate Regan, thinking that rest may improve her condition.
While out on a shoot, Burke Dennings agrees to watch over Regan, who is still under the effects of the powerful sedatives given to her. Chris is horrified when she arrives to find out that Burke is now dead. He had apparently fallen out of the window. The event is assumed to be an accident, as Burke has been known to be a heavy drinker; nonetheless Lt. William investigates his death.
He is accompanied by Fr. Damien Karras, a psychiatrist and Catholic priest who is resuming his duties as a means of coping with the guilt of not having been present during his elderly mother’s death. The doctors recommend to Chris that an exorcism be performed on Regan, as they believe that her condition is mental in nature. Fr. Karras agrees to take up Chris’ case and begins observing Regan.
Regan begins to show even more extreme manifestations of demonic influence: speaking in reverse in an old woman’s voice and scars forming on her abdomen forming the words “Help Me.” These finally convince the reluctant priest that Regan is indeed possessed by an evil spirit. He seeks help to perform the rites and Fr. Merrin is selected to lead as Fr. Karras assists.
The two priests proceed with the exorcism in earnest but the demon Pazuzu toys with them mocking their efforts by having Regan perform a number of offensive and blasphemous acts and threatening them with death. They tie her down to prevent the demon from harming its host. Pazuzu singles out Fr. Karras to torment as his crisis of faith is sensed by the demon and uses it to break his focus. Fr. Merrin dismisses him and attempt to do the rite alone but he dies from exhaustion. Fr. Karras enters the room to try to revive Fr. Merrin but he is too late. The demon begins to berate him, laughing at his grief and helplessness.
Livid, the young priest wrestles the demon possessed Regan to the floor, challenging it to take his body instead and leave Regan alone. The demon storms out of Regan, entering the body of Fr. Karras. Before the demon can subjugate the priest’s will and induce him to harm his charge he flings himself out the window unto the streets below. An old friend and colleague of Fr. Karras, Fr. Dyer, witnesses the tragedy and administers the last rights to his dying friend.
The scene changes to focus once more on Regan, who is now freed of Pazuzu’s influence as they prepare to leave for Los Angeles. Regan mercifully has no memory of her demonic ordeal. She gives Fr. Dyer, who has taken on the MacNeil case upon Fr. Karass tragic death, a kiss on the cheek before they leave. Lt. Kinderman, who now investigates Fr. Karras’ death, has befriended Fr. Dyer to get a better understanding of the mysterious circumstances surrounding the man’s passing.
