In the basement den, Regan - a bright, cheerful, ordinary pre-teen amuses herself with arts-and-crafts materials and paint. She creates an orange, bird-like puppet figure [similar in shape - coincidentally - to the Pazuzu statue]. Her mother discovers a dusty OUIJA board that Regan earlier found in a closet - the theme of communication begins to play out in the film. Lonely and without friends of her own age, and without a partner to play ping-pong in the den, Regan has amused herself by playing with the board:

Chris: Wait a minute, you need two.
Regan: No ya don't. I do it all the time.
Chris: Oh yeah. Well, let's both play.

One of her imaginary friends, 'Captain Howdy' [a play on her father's name, Howard, and an innocuous, innocent name that would obversely connote fear] spins the planchette from under Chris' hands - the first evidence of supernatural telekinesis in the film:

Chris: You really don't want me to play, huh?
Regan: No, I do. Captain Howdy said no.
Chris: Captain who?
Regan: Captain Howdy.
Chris: Who's Captain Howdy?
Regan: You know, I make the questions and he does the answers.

As a smiling, loving Regan is tucked into bed by her mother and they share an intimate conversation, she is reading a recent PHOTOPLAY Magazine with a red-banner cover story: "Big Trouble In the MacNeil Marriage! The Night Howard Walked Out On His Wife." The colorful cover photo depicts Chris with her daughter. After taking the magazine away, Chris teases her daughter about her impending maturity - she is on the verge of becoming an adult:

Chris: Look, Regan, why are you reading that stuff?
Regan: Because I like it.
Chris: It's not even a good picture of you. You look so mature.

Regan's birthday is coming on Sunday - she will be a ripe, pubescent 13 - and they plan for the special sightseeing outing. As an undercutting aside, Regan maturely suggests having her mother's director friend Burke Dennings accompany them - with misleading ideas developing in her head, she promotes a match-making liaison between her mother and Burke:

Regan: You can bring Mr. Dennings if you like...Well, you like him...You're gonna marry him, aren't you?
Chris: Oh God, are you kidding me? Marry Burke Dennings! Don't be silly. Of course not. Where'd you ever get an idea like that? [Captain Howdy may be the source of her information. Howdy and 'Howard,' her father's name, are remarkably similar.]
Regan: But ya like him.
Chris: Of course I like him. I like pizzas too, but I'm not gonna marry one.
Regan: You don't like him like Daddy?
Chris: Regan. I love your Daddy. I'll always love your Daddy, honey. OK? Burke just comes around here a lot, 'cause (she sighs), well, he's lonely. Don't got nutin' to do.
Regan: (slyly) Well, I heard differently.
Chris: Oh you did. What did you hear? Huh?
Regan: I don't know. I just thought.
Chris: Well, you didn't think so good.
Regan: How do you know?
Chris: 'Cause Burke and I are just friends. OK? Really. OK.

A quick cut to the next scene - a noisy bar environment where a jukebox plays an Allman Brothers Band rock tune favorite - 'Ramblin' Man.' Father Karras carries two frothy beer glasses to a table where he is joined by his superior priest Reverend Thomas Bermingham, SJ (himself), president of the university. Clearly juxtaposed with the atmosphere of the dark drinking establishment peopled with 'rebellious' students (who were recently extras on the film set), the young priest finds himself in a troubling situation, heavily weighted down by counseling his fellow priests who feel they are losing their vocation. In despair, inner conflict and guilt, he is worried about his own burdens: his lonely mother and his own loss of faith:

Karras: It's my mother, Tom. She's alone. I never should have left her. At least in New York, I'd be near, I'd be closer.
Tom: Could see about a transfer, Damien.
Karras: I need re-assignment, Tom. I want out of this job. It's wrong. It's no good.
Tom: (reassuring) You're the best we've got.
Karras: Yeah, not really. It's more than psychiatry, and you know that Tom. Some of their problems come down to faith, their vocation and meaning of their lives, and I can't cut it anymore. I need out. I'm unfit. (He drinks from his beer) I think I've lost my faith, Tom.

A low-angle, street-level camera shot of the exterior of the MacNeil's two-story brick house follows the bar scene. Autumn leaves from the bare trees swirl in the wind. Exasperated by an insensitive lack of regard and her own despair over the breakup of the family, Chris berates and swears about her husband for failing to call Regan on her birthday from his hotel in Rome. She hasn't been able to reach him by long-distance for twenty minutes (halfway across the world) and she becomes acutely distressed:

Oh circuits, my ass. He doesn't give a s--t...Operator, don't tell me there's no answer... Would you try it again please and let it ring...Operator, I've given you the number four times...I've been on this f--king line for twenty minutes!

[Difficulties in communication with Regan's father are contrasted with her easy contact with Captain Howdy/Howard.] The camera withdraws from the living room back toward Regan's room, where she stands in her doorway listening to her tormented mother - after hearing more than enough of her cursing mother speaking to her uncaring father, she mutely retreats to her bed where she unties her shoe.

Early the next morning, Chris finds that Regan has joined her in bed, explaining that she is not able to sleep: "My bed was shaking. I can't get to sleep." Regan's mother goes to investigate more mysterious sounds that she hears up in the attic. Finding her way around in the dark with a lit candle, Chris discovers the traps are untriggered and empty - yet the digging noises can still be heard. After being shocked out of her wits in the attic by Karl's sudden appearance and her flaming candlelight, he confirms what she has found: "No rats."

[The next scene was often cited as being one of the most objectionable at the time of the film's release.] On the Georgetown University campus, one of the Jesuit priests discovers that the white marble statue of the Virgin Mary has been desecrated with red paint and other materials, and has taken on the appearance of a harlot. The statue has long, conical red-tipped breasts, red color on both hands, and an elongated, erect penis-shaped clay protuberance also daubed in red. [Blasphemous Black Sabbath rites would often include perversities at the altar or sites of statues.]

There are only faint signs early in the film that Regan is acting strangely, communicating with spirts, and is depressed and anxious. Her mother brings her to a doctor's office for a physical check-up including drawing of a blood sample, an ECG, and a tuning fork test. During her electrocardiogram test, she experiences a momentary vision of a demonic creature. She responds angrily to a doctor examining her with the vibrating tuning fork: "I don't feel anything!", and she refuses to have her temperature taken. She appears dazed as she twirls and hums before collapsing onto the floor in the examining room. Dr. Klein (Barton Heyman) diagnoses Regan's condition as a "disorder of the nerves...often seen in adolescence." He adds that Regan displays all the symptoms, including hyperactivity, a bad temper, and poor performance in mathematics. A prescription is written for the stimulant Ritalin, 10mg each day, to counteract her hyperkinetic behavior. Unofficially, he speculates: "Your daughter's symptoms could be an overreaction to depression" due to the father's recent separation. Chris asks about Regan's frequent attention-getting lies, such as the shaking of her bed, and is told that Regan let loose a string of swear words and obscenities during her exam. He reluctantly provides a detailed quote: "She advised me to keep my fingers away from her goddam c--t."

Inside the corridor of Bellevue psychiatric hospital in New York City, psychiatrist-priest Father Karras has been summoned by his uncle (Titos Vandis) because of his sick mother's hospitalization - she "was all the time screaming, even talking to the radio." [Unable to talk to her relatives, Karras' sick mother communicates with the unseen world of voices on the radio.] Distressed that she was brought to a mental hospital without his permission, he notices deranged, comatose, vacant-eyed women in the hallway and in the psychiatric ward - they exhibit the physical and behavioral symptoms of serious psychological problems. His uncle generates more guilt by berating him. He notes how Damien's choice of the lowly occupation of the priesthood was detrimental to her welfare. His decision to choose God and become a priest rather than enter lucrative private practice as a psychiatrist left her destitute: "If you wasn't a priest, you'd be a famous psychiatrist now...Your mother - she'd be living in a penthouse instead of a ..."

As Father Karras enters the locked ward and walks through to see his mother, the other patients react with agitation and grab at the spiritual figure's clothing. With tears in her eyes, the haggard Mrs. Karras blames her son for her 'imprisonment': "Dimmy, why would you do this to me, Dimmy?" Her thin, frail arms are restrained by straps on the bed. With anger and rage, she struggles to withdraw from him and turn her head away from his comforting hands. After their visit, Karras beseeches his uncle to move her to a different hospital: "Couldn't you put her someplace else?" But that is an impractical solution: "Like what? A private hospital? Who got the money for that, Dimmy?" (A faint screeching sound, similar to the one in the prologue, builds on the soundtrack.) Guilt-ridden, Karras viciously boxes and punches a bag in a gym - he violently and explosively rages against all the demons and turmoils in his own life - angry that he can't afford to treat his mother, and that God has brought illness to his family.

A few nights later, Chris MacNeil throws a big dinner party at her Prospect Street house. Prominent people are among the guests - Burke Dennings and an astronaut (Dick Callinan) guest. Burke becomes visibly drunk, and makes inappropriate remarks to the astronaut: ("There seems to be an alien pubic hair in my Gin. Never seen it before in my life"). He also approaches Chris' housekeeper Karl, carrying a tray of drinks, and spouts derogatory comments about him being a Nazi collaborator working for the Gestapo and Goebbels, and calls him a "Nazi bastard." He follows Karl into the kitchen to continue the taunts: "C--ting Hun! Bloody damn butchering Nazi pig!", triggering Karl to lunge at Burke and strangle him before being pulled away. Soon after, Burke leaves the party with some assistance from Chris and her house-sitter and nanny Sharon Spencer (Kitty Winn) to be chauffeured home.

Regan is giggling and happy while mingling among the party-goers. Chris asks one of her friends, Father Joe Dyer (Reverend William O'Malley, a real-life priest), Father Karras' superior, about St. Michael's next door and the black-haired, "intense-looking" young priest she often sees there. She learns that he's Damien Karras, "our psychiatric counselor. He had a pretty rough knock last night, poor guy. His mother passed away. She was living by herself and I guess she was dead a couple of days before they found her."

Regan is already in bed asleep (and kissed by her mother) before the party is over. But shortly later during a fun sing-along at the piano, Regan comes downstairs and appears during the music-gathering and songfest. She enters the room in her nightgown, and trance-like turns to the astronaut who will soon be launched into space: "You're gonna die up there." The camera frames her feet as she urinates on the carpet-covered floor in front of the guests. Embarrassed and confused, Chris takes her daughter into her arms and apologizes to the astronaut and the rest of the guests:

Regan, oh my God, honey. Honey? Whatsa matter? I'm sorry, she's been sick. She didn't know what she was saying.

Chris helps Regan to retreat upstairs and gives her a hot bath while comforting her. Regan is as puzzled by her own behavior as is Chris. Chris reassures her that she is probably upset because of all the changes she has experienced in the last few months - her father's departure and erratic contact, the new job for Chris, the new town. Regan is promised that everything will be all right:

Chris: What made you say that, Regan? Do you know, sweetheart? (no response from Regan)
Regan: (after the bath in her bedroom) Mother? What's wrong with me?
Chris: It's just like the doctor said. It's nerves, and that's all. OK? You just take your pills and you'll be fine, really. OK?

Returning down the stairs long after the party has ended, Chris finds the housekeeper Willie (Gina Petrushka) scrubbing the stains from the rug. She turns back when she hears Regan screaming in her room and calling for her help. As Chris rushes to her daughter's closed bedroom door [the camera tracks rapidly along with her toward the door] - an electric, candle-shaped light bulb flickers in the hallway. The camera first shows Chris' face registering a horrified, shocking reaction after entering - Regan's bed is racked with violent convulsions. Flopping around on the top of the bed, the young girl frantically calls out: "Mother help me!" Chris throws herself on top of Regan on the wildly bucking bed which bounces up and down on the floor - there is a cacophony of deafening noise equaling all the other loud, grating noises previously heard in the film.

Suddenly, the scene shifts to Father Dyer striding down the corridor of the Jesuit residence hall at Georgetown University, peering into one of the bedrooms where students are smoking, gambling at cards, and drinking. After knocking on D. Karras' door and entering, he brings the priest a bottle of Chivas Regal Scotch Whiskey, joking that he stole it from the college president: "(He) shouldn't drink. It tends to set a bad example. I figured I saved him from a big temptation." Karras is tormented by guilt and anguished remorse for being absent when his mother died, and he begins skeptically doubting his decision to pursue a career as a priest. Dyer encourages Karras to lie back, take off his shoes, and go to sleep.

As Father Karras begins to dream after the lights are turned out, a montage of dream-like images passes and flashes through his consciousness, mixing momentary sights of his mother and her ascent and descent [into death] with all the surrealistic images taken from previous film components accompanying Father Merrin in Iraq: the Christian medallion - now free-falls through the air above a richly-textured Iraqi tapestry, a ferocious, growling desert dog runs toward the camera, Karras' mother stares straight ahead, the pendulum of the curator's clock swings, Karras' mother emerges after ascending from an underground subway in New York City while Karras waves from a traffic island toward his mother, who mother calls out - but doesn't see or heed her son. In addition, a ghoulish, ghostly-white demonic face appears as Karras frantically pursues his mother across traffic on a busy street, and she descends back into the subway entrance. The medallion drops on the hard, stone Georgetown steps.

Regan's scream provides a transition from the dream sequence to the next scene. Chris seeks more medical help and treatment for her daughter, but Regan resists violently: "I don't want it!" She is held down and injected with a sedative by a team of doctors. The girl spits and curses at Dr. Klein: "You f--kin' bastard."

Concurrent with the ministrations of modern medicine, Father Karras, in the University chapel, prays in memory of his deceased mother: "Remember also, Oh Lord, thy servant Mary Karras who has gone before us on the side of faith and sleeps the sleep of peace."

Dr. Klein delivers a second diagnosis to Chris - he explains Regan's strange afflictions and seizures are due to a physical problem - a brain disorder:

It's a symptom of a type of disturbance in the chemical-electrical activity of the brain. In the case of your daughter, in the temporal lobe - it's up here - in the lateral part of the brain. It's rare, but it does cause bizarre hallucinations and usually just before a convulsion...the shaking of the bed. It's doubtless due to muscular spasms.

Chris doubts that her daughter's uncontrollable spasms and body movements caused the bed to buck so violently. The doctor insists her shaking is due to a lesion in her brain:

Chris: Oh no. No, no. That was not a spasm. Look. I got on the bed. The whole bed was thumping and rising off the floor and shaking - the whole thing, with me on it!
Dr. Klein: Mrs. MacNeil, the problem with your daughter is not her bed, it's her brain.
Chris: So, uhm, what causes this...?
Dr. Klein: A lesion. A lesion in the temporal lobe. It's a kind of seizure disorder.
Chris: Now look, Doc, I really don't understand how her whole personality could change.
Dr. Klein: The temporal lobe is very common. It could last for days or even weeks. It isn't rare to find destructive, even criminal behavior.
Chris: Hey, do me a favor, will ya? Tell me somethin' good.
Dr. Klein: Don't be alarmed. If it's a lesion, in a way she's fortunate. All we have to do is remove the scar.

Regan is wheeled on a flat table into a testing area for more examinations designed to unravel the mysteries of her malady. In a long, purposely drawn-out, excruciatingly-torturous sequence with markedly sexual overtones (and a foreshadowing of the eventual exorcism), Regan is prepared by medical assistants for an arteriogram. She is placed on a leather backing, readied for a blood pressure reading, and her shoulders are bared when her hospital gown is tucked under. Electrodes are stuck to her upper arms. One of the male medical technicians, garbed like a priest (in a blue hospital smock) paints her the right side of her pale neck with a "cold and wet" square of cotton dipped in dark brown iodine, an antiseptic. He tests a syringe by squirting [or ejaculating the phallic-shaped object] fluid from its sharp point (seen in large detail), and then 'sticks' her in the cleansed neck area with it to create a small incision. She holds back from squirming and whimpering as he slightly pierces her flesh. With another sharp instrument taken from a trayful of Inquisitional-type torture devices, he warns Regan not to move when she feels "some pressure." He inserts the second device into the incision mark, pushes it in (she gasps as she feels the needle enter), and then releases the cap on the instrument - blood spurts, orgasmically, from the opening in Regan's neck.

The 'deflowering' examination is not complete for the helpless young girl - blood flows through tubes as her chin is taped to the table to keep her stationary. More pieces of medical machinery are wheeled to each side of her head. The lights are dimmed, and she is told to "look up." The positioning of a scanner produces a cross-shaped shadow across Regan's forehead. A button is pushed, causing a tremendous knocking/pounding sound - Regan cringes as pain envelopes her face. The screen turns white for an instant. A full series of X-rays illustrating various death's head angles of Regan's skull are being examined by Dr. Tanney (Robert Symonds) on a white, photo-examination table - loud, whirring gears deliver each set of photographic negative plates for viewing. Tanney pronounces his diagnosis to Dr. Klein as they both examine a full-screen side view of her skull: "There's just nothing there. No vascular displacement at all." To their surprise, they find nothing physically wrong with her.