On a cold autumn day, Chris stands alone on a footbridge, waiting to meet Father Damien Karras who has been recommended by Father Dyer. She abandons her original skepticism about resorting to exorcism (and the diagnosis of demonic possession) by taking her problem to Karras, believing that a religious ritual may be her last hope to save her daughter and drive out the devil. She quizzes him about his background: "How did a shrink ever get to be a priest?" After avoiding the subject for a while, she suddenly asks: "How do you go about getting an exorcism?" He stops short during their walk and asks her to repeat her question: "I beg your pardon?" Karras doesn't believe in exorcisms in the twentieth century and is inclined to doubt demonic possession:

Well, the first thing - I'd have to get into a time machine and get back to the 16th century...Well, it just doesn't happen any more, Mrs. MacNeil...since we learned about mental illness, paranoia, schizophrenia...Since the day I joined the Jesuits, I've never met one priest who has performed an exorcism. Not one.

Chris begs, through choking sobbing, that somebody "very close" to her is "probably possessed and needs an exorcism. Father Karras, it's my little girl." He tries to dissuade her, arguing that the Catholic Church insists on proof that the devil is really in a person: "Then that's all the more reason to forget about exorcism...To begin with, it could make things worse. Secondly, the church before it approves an exorcism conducts an investigation to see if it's warranted. That takes time...I need church approval and that's rarely given." On the other hand, he reluctantly agrees to see her daughter "as a psychiatrist," believing that Regan's horrible descent into hell is a psychiatric illness. But Chris is completely fed up with psychiatrists:

Oh, not a psychiatrist. She needs a priest. She's already seen every f--king psychiatrist in the world and they sent me to you. Now you're gonna send me back to them? Jesus Christ! Won't somebody help me?...Can't you help her, just help her?

Karras is brought to Regan's bedroom to see and talk to Mrs. MacNeil's daughter. Awful sounds emanate from her bedroom as they climb the stairs. In the corridor, Karl explains the child-monster's anger: "It wants no straps." When Karras enters, the girl is strapped to a padded four-poster bed. Her face is cut, her hair matted, her eyes wild-looking, and she has a plastic tube taped to one nostril. The grotesque girl speaks with a disgusting, low-pitched growl coming straight from hell:

Karras: Hello, Regan. I'm a friend of your mothers. I'd like to help you.
Regan: Why not loosen the straps then?
Karras: I'm afraid you might hurt yourself, Regan.
Regan: I'm not Regan.
Karras: I see. Well then, let's introduce ourselves. I'm Damien Karras.
Regan: I'm the devil. Now kindly undo these straps!
Karras: If you're the devil, why not make the straps disappear?
Regan: That's much too vulgar a display of power, Karras.
Karras: Where's Regan?
Regan: In here - with us.
Karras: Show me a Regan and I'll loosen one of the straps.
Regan: (in the voice of the subway bum he has heard before) Can you help an old altar boy, Father? (He turns) Your mother's in here with us, Karras. Would you like to leave a message? I'll see that she gets it.
Karras: If that's true, then you must know my mother's maiden name. What is it? What is it?

In the grossest scene of the film, as he approaches closer for an answer, Regan lurches forward on the bed and spews bilious, pea-green soup vomit from her mouth in a single projectile stream directly into his face. The thick green slime sticks to his face and clothing. Bits of vomit and bile acid also dribble down onto Regan's nightgown.

Chris washes and irons the priest's clothing, as he surveys some of Regan's artwork in the basement. He is again reluctant to get further involved and resort to exorcism: "Look, I'm only against the possibility of doing your daughter more harm than good...I can't do it. I need evidence that the church would accept as signs of possession...like her speaking in a language she's never known or studied...Look, your daughter doesn't say she's a demon. She says she's the devil himself. Now if you've seen as many psychotics as I have, you realize that's the same thing as saying you're Napoleon Bonaparte. You asked me what I think is best for your daughter. Six months, under observation, in the best hospital you can find." His advice is that she needs counseling rather than cleansing. Chris challenges the skeptical Karras to persuade him to believe that the child-monster upstairs is genuinely inhabited by a demon:

I'm telling you that that thing upstairs isn't my daughter. Now I want you to tell me that you know for a fact that there's nothing wrong with my daughter except in her mind. YOU TELL ME YOU KNOW FOR A FACT THAT AN EXORCISM WOULDN'T DO ANY GOOD! YOU TELL ME THAT!

Karras decides to study Regan's condition, unimpressed by her physical manifestations, but amazed at her telepathic knowledge that his mother died. Regan mocks him as he sets up equipment to tape record the many strange voices that seem to be coming from inside her:

Regan: What an excellent day for an exorcism.
Karras: You'd like that?
Regan: Intensely.
Karras: But wouldn't that drive you out of Regan?
Regan: It would bring us together.
Karras: You and Regan.
Regan: You and us. [Here, the demon prophetically foreshadows the climax of the film, when the tormented priest Karras literally takes the demon into himself to free the girl from its power.]

Both a Jesuit priest and a psychiatrist by training, Karras goes about various tests to determine whether Regan's case of demon possession is authentic. He finds her powers of telekinesis (opening a nightstand drawer without touching it) unusual, but random. Remarkably, the demon uses Regan as a mouthpiece to speak Latin and French. Sprinkling 'holy water' over Regan in a cross-like pattern causes the Regan-demon to squirm and squeal with extreme fear at the Christian artifact: "It burns!" Diabolical sounds emanate from her mouth - growling dogs, squealing pigs, rasping groans, and foul language. Later, Karras tells Chris that it wasn't 'holy water' but ordinary 'tap water' - "Holy water's blessed, and that doesn't help support a case of a possession." Chris confesses, in a whisper, Regan's complicity in Dennings' murder: "She pushed him out her window."

Karras takes his tape recording to a sound expert - who after playing it declares that it is English in reverse: "It's a language all right. It's English...in reverse." Regan's deep voice calls out the priest's name "Merrin" over and over again. Karras is again summoned to the MacNeil house and brought upstairs by Sharon. She secretly confides: "I don't want Chris to see this." With a flashlight, they creep into the room - a freezing cold place where the sleeping Regan has reduced the temperature at will. Sharon peels back the blankets and opens Regan's nightshirt - the skin on Regan's abdomen has raised welts that form scarring words: "help me." All that is left of the real Regan strives to communicate and pleads for relief.

Persuaded and half-convinced that an exorcism must be performed to scourge the offending demon, Karras requests permission from his superior to proceed with Regan's case: "...I have made a prudent judgment that it meets the conditions set down in the ritual." It is recommended that an older priest, someone with prior experience of an exorcism, be chosen as the exorcist - "maybe someone who has spent time in foreign missions." Archaeologist-priest Father Lankester Merrin (who has returned from the site dig three or four months earlier and is writing a book in Woodstock) is chosen to be the skilled exorcist, and Karras is appointed as his assistant.

To provide a foreshadowing of the danger involved in casting out demons, it is remembered that ten to twelve years earlier while touring in Africa, Merrin conducted an exorcism that lasted months: "...heard it damn near killed him." [Ironically, Merrin's inadvertent unleashing of the demon in Iraq caused the crisis in the first place, and also caused the demon to enter Regan - to exact revenge.] Appropriately, Merrin is summoned to perform the exorcism and battle the evil demon.

On a dark foggy night, in one of the most memorable images of the film - its trademark, Father Merrin arrives by cab at the MacNeil home. He stands motionless under the streetlight in the swirling smoke and looks up at Regan's window - a shaft of bright light emanates down in a broad swatch. [Iconic images are paradoxically reversed - the 'good' priest is black and haunting, and the 'evil' demonic force casts a bright, illuminating light.] A full-closeup of Regan's face - with cat-like eyes, senses an old enemy. Merrin's dark silhouette appears at the doorway. As Merrin enters the house and greets Karras, the dark spirit residing in the girl cries a long, drawn-out curse: "Merrin!" Karras first wishes to explain the manifestations and background of the case, but Merrin wants to begin immediately.

With special gifts and experience, the austere Merrin prepares the younger priest with cautious advice. They must avoid conversations with the demon and not listen to the demon's voice because "the demon is a liar and would like to confuse us. But he will also mix lies with the truth to attack us. The attack is psychological, Damien, and powerful...Remember that. Do not listen." Father Karras begins explaining Regan's three different personalities - Father Merrin rudely explains that there is only one manifestation inside Regan - Pazuzu:

Karras: I think it might be helpful if I gave you some background on the different personalities Regan has manifested. So far, I'd say there seem to be three. She's convinced...
Merrin: (brusquely) There is only one.

In the dramatic finale, the two men enter Regan's ice-cold bedroom, prepared to do spiritual battle. Garbed in priestly outfits, they also bring weapons of the spirit for exorcism - holy water, holy texts, and a crucifix. The devil's voice emanates from the demonic, staring, fixed-eyes visage of Regan. It curses at Father Merrin, as he recites holy scripture, with the foulest epithet in the film:

Stick your c--k up her ass, you mother-f--king worthless c--ks--ker.

Merrin splashes her body with holy water and yells back: "Be silent!" Regan screams and squirms away, twisting in pain as if burned by the sanctified water. They begin to conduct a RITE OF EXORCISM from red prayer books and recite the Lord's Prayer. The possessed child-monster spews yellowish slime onto Merrin's face. As they desperately pray and the temperature drops in the room, Regan reacts with head-rolling, more writhing, shrieking and vicious growling, sprinkled with more obscene vulgarities to taunt Karras:

Your mother sucks c--ks in hell, Karras.

The demon inside Regan struggles with her arm restraints and bucks the bed legs off the floor. The entire bed levitates into mid-air, pulled upwards by Regan's super-human strength. Regan's flopping head is momentarily replaced with the ghoulish, vampirish, white-faced image seen earlier in Karras' nightmare.

The monstrous battle continues as the bed sinks back in place. Regan's horrible, purple tongue darts in and out of her mouth. As Merrin places his purple cloth stole on her to bless her, a greenish vomit oozes out of her mouth to defile it. The demon fights with the holy men by violently opening and closing cabinet doors. Reaching his own limits, Merrin curses the demon:

Merrin: I cast you out! Unclean spirit in the name of our Lord Jesus Christ.
Regan: Shove it up your a--, you f--kin'...

The walls and ceiling crack, the intravenous medical bottle apparatus crashes to the floor in pieces, and the bathroom door slams and splits as Merrin continues to cast out the demon for possession of her body and soul. He traces crosses on her temple: "Be gone from this creature of God. Be gone, in the name of the Father and of the Son, and the Holy Spirit. With this sign of the holy cross of our Lord Jesus Christ, who lives and reigns with the Father and the Holy Spirit." She sits upright and her head turns a full circle - 360 degrees. Regan's face, in a full face closeup, becomes eerily animated. The demonic beast accuses Karras of murder: "You killed your mother. You left her alone to die." The room violently shakes and the straps on Regan's wrists slowly rip open. Her milky-white eyes roll demonically backwards - a reminder of the blind eye of the steel worker in the prologue.

Her body levitates off the bed toward the ceiling, and her fully-extended body hovers in mid-air - her outstretched arms take the form of a cross. Merrin interprets: "It's the power of Christ. The power of Christ compels you." They chant the phrase repeatedly, and sprinkle more holy water - producing reddish-raw sores on her skin. She slowly sinks back down onto the bed. Karras binds her wrists - she retaliates by striking him from behind. The forces of the demon are unleashed - a back-lit demonic Pazuzu statue appears behind her.

Exhausted by the disgusting ordeal, the two priests rest before starting again. They stagger out of the bedroom, and the frail Father Merrin takes his much-needed heart medicine. Karras returns to the bedroom where Regan has transformed herself into a vision of his mother seated upright on the bed and wearing a white nightgown. As he wipes Regan's forehead, she speaks in his mother's voice to taunt him: "Dimmy, why did you do this to me? Please Dimmy, I'm afraid." He is tormented by the likeness to his mother's voice and screams: "You're not my mother!" Merrin, who has returned to the bedroom, cautions him to leave the bedroom: "Don't listen...Get out!"

Downstairs, Chris asks Karras about their progress: "Is it over?" and "Is she going to die?" He answers in a determined tone before returning upstairs: "No." The doorbell sounds (it signals the arrival of Lt. Kinderman) - another portent of bad news. When Karras joins Father Merrin in the bedroom, it is too late - the old priest is slumped over the bed, dead of a heart attack. Regan is sitting up against one of the padded bedposts - smiling and giggling. The enraged priest assaults her and throws her to the floor, shouting: "You son-of-a-bitch!" He punches her repeatedly in the face with his fist and tries to kill her.

In a supremely self-sacrificial act during the cathartic finale - an indication that he has regained his own faith through his contact with Father Merrin and by the undeniable realization that the Devil really exists, the formerly-rebellious priest Karras taunts the demon inside Regan. He provokes and welcomes the demon to leave her body and come into his own so that he can destroy the Evil:

Take me. Come into me. God-damn you. Take me. Take me.

She grabs and rips the amulet medal from his neck - symbolically removing his protection from evil (foreshadowed previously in Damien's surreal dream). At the moment of his own demonic possession, he suddenly pulls back, his body trembles and his eyes roll up, and his face momentarily takes on the appearance of Regan's demon - he growls and tumbles backwards. On the floor, Regan has regained her former self, and her stifled cries are made in her own voice, but she is terrorized by the demon within Karras. Now that he is filled with the beast-monster, he stands and staggers toward her with his arms outstretched to strangle her - but with all his own fortitude and strength, he screams: "No!" as he battles the demon's attempt to kill her. He hurls himself toward the bedroom window - his body is thrown through the glass and he falls to his death on the steep concrete steps below. [Due to the film's popularity, the steep stairway at the end of M Street in Georgetown has since become a minor tourist attraction.] Karras [with a symbolic first name - Damien/Demon] gives his own life to save Regan's spirit and life, with the promise of being reborn.

From the bedroom window, Lt. Kinderman views Karras' bloodied body ["FIGHT PIGS," a slogan typical of early 70's rebellion, appropriately adorns one of the adjoining walls of the steps, in red] at the foot of the Prospect Street steps. Regan cries hysterically, but she is cured. With police cars and bystanders crowding around, Father Dyer breaks through and grabs Karras' hand, beseeching him: "Do you want to make a confession? Are you sorry for having offended God and for all the sins of your past life?" Signaling his assent, Karras unclenches and grips Dyer's hand. Dyer absolves him of his sins during the administration of last rites. The price or cost of Regan's recovery to sanity and wholeness is that both priests die during the exorcism.

A few weeks later, the stairway steps are cleaned up, and Karl packs luggage in the car for the MacNeil's return to their home in Los Angeles. The house is being packed for the move. As Sharon bids Chris a final goodbye, she hands her the amulet medal (the silver medallion that Father Merrin found in Iraq) that she found in Regan's bedroom. In the driveway, Chris meets Father Dyer, telling him that Regan mercifully remembers nothing of the ordeal. She has repressed the experience entirely:

Chris: She doesn't remember any of it.
Father Dyer: That's good.

A normal, cheerful, healthy-looking pre-teen restored to her former self, but still with bruises on her face - joins them. After she sees Father Dyer's clerical collar, she kisses the Father's cheek as if to say - thank you. Chris stops the car after they have driven a block away and engages in a brief exchange with the Father. She gives him the amulet medal as a remembrance and parting gift: "I thought you'd like to keep this." His cupped hand accepts and encloses the medal as they drive away.

Dyer solemnly pauses at the top of the stairs, just below the boarded up window of Regan's bedroom.

The film ends on a chilling note with Dyer turning away from the steps, the sounds of "Tubular Bells," and an orchestral chord of emphasis - with the jolting view of the red-on-black title: "THE EXORCIST."