Chris calls the two doctors at the lab to urgently summon them to her Prospect Street home. The ringing of the doorbell at the MacNeil home overlaps their departure from the hospital - and signals the upcoming terror. Chris' secretary/assistant Sharon Spencer rushes down the stairs to answer the door and let the doctors in. From behind, Regan's hysterical, despairing screams are heard and "things have gotten worse...they've gotten violent." A hand-held camera tracks in front of the group as they hurriedly rush upstairs, down the corridor, and up to Regan's sealed bedroom door. Sharon announces: "Chris! Doctors!" and they are admitted. Within the bedroom, the camera faces back toward the opened door, registering again the fear on everyone's faces.

Regan's upper torso is violently being thrashed, whipped and thrown back and forth on the bed, battering her body as it slams into the mattress. She screams: "Oh please, Mother, make it stop! It's hurting." Then she is tossed upwards and bounces up and down. Her uncontrollable seizures are accompanied by low gutteral growls, almost animalistic. Her throat below her chin abnormally bubbles out.

When Dr. Klein reaches for Regan on the bed, she slaps him back-handed across the face, knocking him onto the floor. Her physically-repulsive voice warns: "Keep away! The sow is mine!" She pulls up the front of her nightgown, pantomimes rubbing herself, and in a deep, strange voice, beckons: "F--k me! F--k me!" [Note: The demon's gutteral voice is that of Mercedes McCambridge.] She flails and thrashes around on the bed, afflicted and possessed by some fantastic force.

While she is forcibly held tight and restrained, a doctor removes another syringe and injects her with a sedative to calm her down. Sharon drags Chris from the scene to remove her from the horrifying cries of her daughter. When the bedroom door is slammed, there is an eerie quiet in the corridor. Sharon and Chris sit huddled together. The doctors Tanney and Klein emerge from Regan's now-quiet bedroom: "She's heavily sedated. She'll probably sleep through tomorrow." Chris demands answers from them: "What was going on in there? How could she fly off the bed like that?" They are still convinced of a physical ailment and propose further tests:

Doctor Tanney: Pathological states can induce abnormal strength. Accelerated motor performance. Now, for example, say a 90 pound woman sees her child pinned under the wheel of a truck. Runs out and lifts the wheels a half a foot up off the ground - you've heard the story - same thing here. Same principle, I mean.
Chris: So what's wrong with her?
Doctor Klein: We still think the temporal lobe.
Chris: (hysterically) Oh what are you talking about, for Christ's sakes. Did you see her or not? She's acting like she's f--king out of her mind, psychotic, like a... split personality or ...
Doctor Tanney: There haven't been more than a hundred authentic cases of so-called split personality, Mrs. MacNeil. Now I know the temptation is to leap to psychiatry. But any reasonable psychiatrist would exhaust the somatic possibilities first.
Chris: So, what's next?
Doctor Tanney: A pneumoencephalogram, I would think. Pin down that lesion. It will involve another spinal.
Chris: Oh, Christ!
Doctor Tanney: What we missed in the EEG and the arteriograms could conceivably turn up there. At least, it would eliminate certain other possibilities.

Regan is submitted to other high-tech, painful tests and diagnostic procedures, including a pneumoencephalogram - an X-ray of the brain made by replacing spinal fluid with a gas (often oxygen) to improve contrast. Her sweating face strains as she is subjected to a high-pitched scanning machine that circles overhead. Klein and Tanney find the newest X-rays are "negative - in other words, normal." There is nothing wrong with Regan's brain. Klein asks Chris suspiciously if drugs may be a factor: "Do you keep any drugs in your house?" Chris reveals that she won't be moving to L.A. right away. She is "building a new house - the old one's been sold. I was gonna take (sniffling)...Regan to Europe for a while after she finished school." Klein suggests: "It's time we started looking for a psychiatrist."

After consulting with the doctor, Chris drives home that evening with her headlights beaming through the dark, she passes red-lighted emergency vehicles (with sirens blaring) at the base of the Prospect Street stairs. The lights in her kitchen flicker and then go off as she answers the loud-ringing phone (with no caller). Upstairs, she finds Regan asleep in her freezing cold bedroom - her breath is visible in the air. The windows are wide open and the wind-blown curtains are billowing outward. She covers Regan with blankets and then proceeds downstairs in a furious mood. Chris lambasts Sharon for leaving Regan alone with her windows wide open. Sharon explains, as the doorbell rings incessantly - another signal of approaching doom, that when she went to get Regan's thorazine medication, Burke remained to stay with Regan. She apologizes:

Sharon: I should have known better. I'm sorry.
Chris: Yeah, I guess you should have.
Sharon: How were the tests?
Chris: We have to start looking for a shrink.

Chuck (Ron Faber) who works with Chris and Burke as assistant director, is let in from the front porch, bringing bad news. Burke was found dead (with a broken neck) at the bottom of a very steep and long flight of stairs (just outside Regan's bedroom window):

Chuck: I suppose you heard.
Chris: Heard what?
Chuck: You haven't heard. Burke's dead. He must have been drunk. He fell down from the top of the steps right outside. By the time he hit "M" Street, he broke his neck.

Chris turns away in horror, shocked by the revelation. She pounds on the wall with her fists. The screen turns dark - a short remembrance for another victim.

[Note: In The Exorcist: The Version You've Never Seen (2000), in addition to other supplemental scenes and additions, at this point in the film, Regan swiftly and quietly descends on all fours down the stairs, upside-down with blood dripping from her mouth, to everyone's horror.]

A voice commands: "When I touch your forehead, open your eyes." The screen returns - Regan's face is being lightly touched by a psychiatrist (Arthur Storch), one of the new practitioners who begins groping for a more accurate assessment of Regan's problems. She has been put in an hypnotic trance (seated in a living room chair in her house) for questioning by the sweet-voiced psychiatrist:

Psychiatrist: How old are you?
Regan: Twelve.
Psychiatrist: Is there someone inside you?
Regan: Sometimes.
Psychiatrist: Who is it?
Regan: I don't know.
Psychiatrist: Is it Captain Howdy?
Regan: I don't know.
Psychiatrist: If I ask him to tell me, will you let him answer?
Regan: No!
Psychiatrist: Why not?
Regan: I'm afraid.
Psychiatrist: If he talks to me, I think he'll leave you. Do you want him to leave you?
Regan: Yes.
Psychiatrist: (he stands and addresses Regan's tormentor): I'm speaking to the person inside of Regan now. If you are there, you too are hypnotized and must answer all my questions. Come forward and answer me now.

A black and white framed photograph of Regan inexplicably falls forward off the mantle. Regan face contorts and a low, wolf-like growl emanates from her mouth. The hypnotist continues, thinking he is speaking to the "someone" inside her: "Are you the person inside of Regan? Who are you?" To abortively end the session, Regan slams her hand into the psychiatrist's crotch, grabs him and painfully crushes his genitals. Then her fury rises as she falls on top of the hapless examiner. She must be dragged off and held.

Middle-aged homicide detective Lt. William Kinderman (Lee J. Cobb) [his name translates as "child man" in German], assigned to the case following Burke Dennings' death and the defilement of the Virgin Mary statue, meets with Father Karras as he finishes up a ran around the George University outdoor track. As they stroll through the campus, he questions Father Karras about the subject of witchcraft, already aware that the priest had written an authoritative paper on the subject "from the psychiatric end." Kinderman suspects ritualistic overtones - Dennings was possibly killed by having his head twisted around in a ritualistic black-magic murder. The detective suggests that the priest identify any of those he counsels who might be capable of committing the murder:

Kinderman: Well, this desecration in the church. Do you think this has anything to do with witchcraft?
Karras: Maybe. Some rituals use the Black Mass. Maybe.
Kinderman: And now, Dennings, you read how he died?
Karras: In a fall.
Kinderman: Let me tell ya how, and please Father, confidential. Burke Dennings, good Father, was found at the bottom of those steps leading to M Street with his head turned completely around - facing backwards.
Karras: It didn't happen in the fall?
Kinderman: It's possible. Possible, however -
Karras: ...unlikely.
Kinderman: Exactly. So on the one hand, we've got a witchcraft kind of murder, and on the other hand a Black Mass type desecration in the church.
Karras: You think the killer and the desecrator are the same?
Kinderman: Maybe somebody crazy. Somebody with a spite against the church. Some unconscious rebellion.
Karras: A sick priest - is that it?
Kinderman: Look, Father, this is hard for you. Please, I understand, but for priests on the campus here, you're the psychiatrist. You know who was sick at the time, who wasn't. I mean, this kind of sickness. You'd know that.
Karras: I don't know anyone who fits that description.
Kinderman: Ah! Doctor's ethics! If you knew, you wouldn't tell, huh?
Karras: No, I probably wouldn't.
Kinderman: Not to bother you with trivia, but a psychiatrist, in sunny California no less, was put in jail for not telling the police what he knew about a patient.
Karras: Is that a threat?
Kinderman: No, I mention it only in passing.
Karras: Incidentally, I mention it only in passing. I could always tell the judge it was a matter of confession.

At the Barringer Clinic and Foundation, psychiatrists conclude that Regan's symptoms are evidence of a rare disorder hardly seen anymore except in primitive cultures. Regan, with cuts marks on her face and dehydrated lips, struggles in a clinic bed, strapped down like Mrs. Karras was earlier at Bellevue. Her ailments are similar to a form of possession: "Quite frankly, we really don't know much about it at all except that it starts with a conflict or a guilt and it leads to the patients' delusions that his body has been invaded by some alien intelligence - a spirit if you will." After 88 doctors, Chris despairs, refusing "to lock her up in some god-damned asylum." She is fed-up with their "bulls--t" and medical-babble.

Because Chris has no specific religious beliefs, she is told by a panel of psychiatrists led by the Clinic's Director Dr. Barringer (Peter Masterson), that religious counsel - an exorcism - might cure Regan and rid her of her possession:

Doctor: There is one outside chance for a cure. I think of it as shock treatment - as I said, it's a very outside chance...Have you ever heard of exorcism? Well, it's a stylized ritual in which the rabbi or the priest try to drive out the so-called invading spirit. It's been pretty much discarded these days except by the Catholics who keep it in the closet as a sort of an embarrassment, but uh, it has worked. In fact, although not for the reasons they think, of course. It's purely a force of suggestion. The victim's belief in possession is what helped cause it, so in that same way, a belief in the power of exorcism can make it disappear.
Chris: (uneasily) You're telling me that I should take my daughter to a witch doctor? Is that it?

Regan is brought home from the clinic, while Lt. Kinderman surveys the Dennings death scene at the foot of the stairs. As Chris tucks Regan in her bed, she simultaneously discovers a crucifix under her pillow - possibly put there by the religiously-superstitious servants or babysitter, but all of them flatly deny it. Dramatically timed, Kinderman also finds a small clay, pig-faced talisman [resembling the Pazuzu amulet found in Iraq] near the steps. At the top of the stairs, Kinderman looks up at the MacNeil's bedroom window nearby.

Kinderman meets with Chris for coffee, finding ominous signs in his investigation and immediately alarming Regan's nervous mother. It dawns on her that it was Regan who had killed Dennings:

Kinderman: It's strange. The deceased comes to visit - stays only 20 minutes. And leaves all alone a very sick girl. And speaking plainly, Mrs. MacNeil, it isn't likely he would fall from a window. Besides, a fall wouldn't do to his neck what we found, except maybe one chance in a thousand. Nope, my hunch, my opinion - he was killed by a very powerful man - point one. And the fracturing of his skull - point two, plus the various other things we mentioned would make it very probable, probable, not certain, that the deceased was killed and then pushed from your daughter's window. But nobody was in the room, except your daughter. So how can this be? It could be one way. If someone came calling between the time Miss Spencer left and the time you returned...
Chris: Judas Priest. Just a second.

Chris knows that bedridden Regan was the only one in the house with Burke just before his death. Kinderman notices that small sculpted animals in the MacNeil house - made by Regan - match the one he had just found at the base of the stone stairway: "Your daughter? She's the artist?" He also probingly asks for a favor: "Incidentally, uhm, you might ask your daughter if she remembers seeing Mr. Dennings in her room that night?" After Kinderman leaves, Chris ineffectually slips the lock on the front door behind him.

One of the film's most horrifying scenes occurs next - the notorious crucifix-masturbation scene, symbolically simulating the loss of virginity for the young teenager. From upstairs in Regan's bedroom, Chris hears grotesque sounds, crashes, and screams. She runs up the stairs towards the door - it opens and she sees 45 rpm records, books, and stuffed animals being hurled at the tightly-closed window. The camera registers Chris' horror on her face as she sees her daughter's sacrilegious self-abuse. In an obscene gesture simulating masturbation, a horribly-disfigured Regan repeatedly thrusts her bloodied hand clutching the crucifix into her vagina under her blood-splattered nightgown, as she bellows obscenities in the Devil's voice: "Let Jesus f--k you, let Jesus f--k you! Let him f--k you!" [The demon's voice was enhanced with various animal noises and other grotesque sounds.]

Chris grabs her daughter's super-strong arm and tussles with her for control of the offending object. Regan holds her mother's head down into her crotch and repeats: "Lick me!" - covering her mother's face in blood. Then, Regan punches her mother with a violent blow, sending her backwards across the bedroom floor. With her telekinetic power, Regan moves a chair against the door to bar the way of Sharon and others, and she sends a tall wooden bureau across the floor toward her sprawled mother. As a bloody-faced Regan sits on her bed, she spins her head backwards 180 degrees, threatening in a deep malevolent voice as she imitates Dennings' British accent (and his manner of death) to taunt Chris about his murder: "Do you know what she did? Your c--ting daughter?"