the exorcist

Based on the 1971 novel by William Peter Blatty, The Exorcist marries three different scenarios into one plot.
The movie starts with Father Lankester Merrin (Max von Sydow) on an archaeological dig near Nineveh. He is then brought to a nearby hole where a small stone head is found, resembling some sort of creature. After talking to one of his supervisors, he then travels to a spot where a strange statue stands, specifically Pazuzu, with a head similar to the one he found earlier. He sees an ominous man up a bit away, and two dogs fight loudly nearby, setting the tone for the rest of the film.
Meanwhile, Father Damian Karras, a young priest at Georgetown University, begins to doubt his faith while dealing with his mother's terminal sickness.
In the central storyline, Chris MacNeil (Ellen Burstyn), an actress filming in Georgetown, notices dramatic and dangerous changes in the behavior and physical make-up of her 12-year-old daughter Regan Theresa MacNeil (Linda Blair). Regan exhibits strange, unnatural powers, including levitation and great strength. At first, Chris believes that Regan's rapid mental and physical changes are due to trauma from Chris's recent divorce. Regan is forced to endure a series of unpleasant medical tests as doctors try to find an explanation for her bizarre changes. During this time, several supernatural occurrences plague the household of the MacNeils, including violently shaking beds, strange noises and unexplained movement. When all medical possibilities of explaining Regan's worsening condition are exhausted, a doctor recommends an exorcism, explaining that if Regan's symptoms are a psychosomatic result of a belief in demonic possession, then an exorcism would likewise have the psychosomatic effect of ending such symptoms.

Chris consults Father Karras, since he is both a priest and a psychiatrist. Despite an initial conclusion that Regan's problems are psychological, Damian is eventually convinced that Regan is possessed, after witnessing otherwise unexplainable events.

Father Merrin, who in addition to being an archeologist is also experienced in exorcism, is summoned to Washington. He and Father Karras try to drive the spirit from Regan before she dies.[2] Regan, or rather the spirit, claims she is not possessed by a simple demon, but by the Devil himself.
At the climax of the exorcism, Father Merrin dies of heart failure and Father Karras shouts at the demon to enter him. The demon does enter Damian, but the priest immediately throws himself through Regan's bedroom window in order to stop the spirit from murdering Regan, killing himself in a fall down the steps outside. Regan is restored to her normal self, and according to Chris, claims she does not remember any of the experience. The film ends as the MacNeil mother and daughter return to Los Angeles to move on from their ordeal.