When a teenage girl is possessed by a mysterious entity, her mother seeks the help of two priests to save her daughter.

Demon: What an excellent day for an exorcism.
Father Damien Karras: You would like that?
Demon: Intensely.
Father Damien Karras: But wouldn't that drive you out of Regan?
Demon: It would bring us together.
Father Damien Karras: You and Regan?
Demon: You and us.
Demon: I'm not Regan.
Father Damien Karras: Well, then let's introduce ourselves. I'm Damien Karras.
Demon: And I'm the Devil. Now kindly undo these straps.
Father Damien Karras: If you're the Devil, why not make the straps disappear?
Demon: That's much too vulgar a display of power, Karras.
Demon: Your mother sucks cocks in Hell, Karras, you faithless slime.
[repeated lines]
Father Merrin, Father Damien Karras: The Power of Christ compels you!
Father Merrin: Especially important is the warning to avoid conversations with the demon. We may ask what is relevant but anything beyond that is dangerous. He is a liar. The demon is a liar. He will lie to confuse us. But he will also mix lies with the truth to attack us. The attack is psychological, Damien, and powerful. So don't listen to him. Remember that - do not listen.
Demon: Stick your cock up her ass, you motherfucking worthless cocksucker.
Psychiatrist: Is there someone inside you?
Regan MacNeil: Sometimes.
Psychiatrist: Who is it?
Regan MacNeil: I don't know.
Psychiatrist: Is it Captain Howdy?
Regan MacNeil: I don't know.
Psychiatrist: If I ask him to tell me, will you let him answer?
Regan MacNeil: No.
Psychiatrist: Why not?
Regan MacNeil: I'm afraid.
Demon: Mirabile dictu, don't you agree?
Father Damien Karras: You speak Latin?
Demon: Ego te absolvo.
Father Damien Karras: Quod nomen mihi est?
Demon: Bon Jour.
Father Damien Karras: Quod nomen mihi est?
Demon: La plume de ma tante.
Father Damien Karras: Why her? Why this girl?
Father Merrin: I think the point is to make us despair. To see ourselves as... animal and ugly. To make us reject the possibility that God could love us.
Father Damien Karras: [sees Father Merrin dead on Regan's bed and gently moves him on the floor]
Regan MacNeil: [looks at Karras and Merrrin]
Father Damien Karras: [beats with great force on Merrin's breast, checks whether he can hear Merrin's heartbeat]
Demon: [fakes crying]
Father Damien Karras: [enrages] You son of a bitch!
[beats Regan several times]
Demon: [roars]
Father Damien Karras: [almost strangles Regan] Take me! Come into me! God damn you! Take me! Take me!
Father Damien Karras: [feels the demon entering him]
Regan MacNeil: [freed from the demon, starts to cry]
Father Damien Karras: [falls down, is about to kill Regan, but screams:] NOOOOO!
[jumps out of the window and deadly rolls from the stairs near the house]
Burke Dennings: [to Karl] Cunting Hun! Bloody damn butchering Nazi pig!
Father Damien Karras: Where is Regan?
Demon: In here. With us.
[2000 version]
Chris MacNeil: Well, give me an example. Like what specifically did she say?
Dr. Klein: Specifically, Mrs. MacNeil, she advised me to "keep my fingers away from her goddamned cunt."
Demon: Do you know what she did, your cunting daughter?
Father Merrin: I cast you out! Unclean spirit!
Demon: Shove it up your ass, you faggot!
Father Merrin: In the Name of our Lord Jesus Christ! It is he who commands you! It is he who flung you from the gates of Heaven to the depths of Hell!
Demon: Fuck him!
Father Merrin: Be gone...
Demon: Fuck him, Karras! Fuck him!
Father Merrin: ...from this creature of God! Be gone! In the Name of the Father, the Son, and the Holy Spirit!
Father Damien 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...
Father Merrin: There is only one.
Regan MacNeil: You're going to die up there.
Dr. Barringer, Clinic Director: It looks like a type of disorder that you rarely ever see any more, except in primitive cultures. We call it a somnambuliform possession. Quite frankly, we don't know much about it except that it starts with some conflict or guilt that eventually leads to the patient's delusion that his body's been invaded by an alien intelligence, a spirit if you will.
Chris MacNeil: Look, I'm telling you again and you'd better believe it: I'm not about to put her in a goddamn asylum! And I don't care what you call it! I'm not putting her away!
Dr. Barringer, Clinic Director: I'm sorry.
Chris MacNeil: You're sorry! Jesus Christ, 88 doctors and all you can tell me with all of your bullshit is...
Dr. Barringer, Clinic Director: There is one outside chance of a cure. I think of it as shock treatment. As I say, there is an outside chance...
Chris MacNeil: Will you just name it, for God's sake? What is it?
Male Doctor: Do you have any religious beliefs?
Chris MacNeil: No.
Female Doctor: What about your daughter?
Chris MacNeil: No, why?
Dr. Barringer, Clinic Director: Have you ever heard of exorcism? It's a stylized ritual in which rabbis or priests try to drive out the so-called invading spirit. It's pretty much discarded these days, except by the Catholics who keep it in the closet as a sort of embarrassment. It has worked, in fact, although not for the reason they think, of course. It was purely the force of suggestion. The victim's belief in possession helped cause it. And just in the same way, this belief in the power of exorcism can make it disappear.
Karras' Mother: [to Karras] Why you do this to me, Dimmy?
[Regan, possessed, is masturbating with a crucifix]
Demon: Let Jesus fuck you, let Jesus fuck you. Let him fuck you.
Father Damien Karras: There are no experts. You probably know as much about possession than most priests. Look, your daughter doesn't say she's a demon. She says she's the devil himself. And if you've seen as many psychotics as I have, you'd know it's like saying you're Napoleon Bonaparte.
Demon: Your mother's in here, Karras. Would you like to leave a message? I'll see that she gets it.
Chris MacNeil: [as the Ouija planchette pulls away] You really don't want me to play, huh?
Regan MacNeil: No, I do. Captain Howdy said no.
Chris MacNeil: Captain who?
Regan MacNeil: Captain Howdy.
Chris MacNeil: Who's Captain Howdy?
Regan MacNeil: You know, I make the questions and he does the answers.
Chris MacNeil: Oh, Captain Howdy...
Chris MacNeil: What are you doing here?
Regan MacNeil: My bed was shaking. I can't get to sleep.
Burke Dennings: Tell me, was it public relations you did for the Gestapo or community relations?
Karl: I'm Swiss!
Burke Dennings: Oh, of course. And you never went bowling with Goebbels before either, I suppose? Nazi bastard.
[2000 version]
Father Merrin: [on his way to begin the exorcism] What is your daughter's middle name, Mrs. MacNeil?
Chris MacNeil: Teresa.
Father Merrin: What a lovely name.
Regan MacNeil: Mother? What's wrong with me?
Chris MacNeil: It's just like the doctor said. It's nerves, and that's all. You just take your pills and you'll be fine, really. Okay?
[first lines]
Boy: [in Arabic] They've found something... small pieces.
Demon: Lick me, lick me!
Subway Vagrant: Father, could you help an old altar boy? I'm Cat'lick.
Demon: Keep away. The sow is mine.
Dr. Barringer, Clinic Director: 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 MacNeil: You're telling me that I should take my daughter to a witch doctor? Is that it?
Regan MacNeil: But ya like him.
Chris MacNeil: Of course I like him. I like pizzas too, but I'm not gonna marry one.
Demon: You killed your mother! You left her alone to die! Bastard!
Father Damien Karras: Shut up!
Regan MacNeil: Captain Howdy, do you think my mom's pretty? Captain Howdy? Captain Howdy, that isn't very nice!
Chris MacNeil: Well, maybe he's sleeping.
Dr. Klein: Do you keep any drugs in your house?
Chris MacNeil: No, of course not, nothing like that.
Dr. Klein: Are you sure?
Chris MacNeil: Well, of course I'm sure. I'd tell you. Christ, I don't even smoke grass.
[2000 version]
Lt. Kinderman: You go to films, Father Dyer? You like them?
Father Dyer: Oh, sure.
Lt. Kinderman: I get passes. In fact I've got a pass for the Crest tomorrow night. You'd like to go?
Father Dyer: What's playing?
Lt. Kinderman: "Wuthering Heights".
Father Dyer: Who's in it?
Lt. Kinderman: Heathcliff, Jackie Gleason, and in the role of Catherine Earnshaw, Lucille Ball. You're happy?
Father Dyer: I've seen it.
Lt. Kinderman: Another one.
Father Dyer: My idea of Heaven is a solid white nightclub with me as a headliner for all eternity, and they *love* me.
[last lines - original version]
Chris MacNeil: Father Dyer? I thought you'd like to keep this.
Chris MacNeil: You show me Regan's double, same face, same voice, everything. And I'd know it wasn't Regan. I'd know in my gut. And I'm telling you 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 for a fact that an exorcism wouldn't do any good. You tell me that!
Chris MacNeil: Operator, you've given me the number four times. What did you do, take an illiteracy test to get that job for Christ sake?
Chris MacNeil: We've got rats in the attic. You better get some traps.
Karl: Rats?
Chris MacNeil: Mm-hmm. 'Fraid so.
Karl: But the attic is clean.
Chris MacNeil: All right, then we've got clean rats.
Father Damien 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, President of University: Could see about a transfer, Damien.
Father Damien Karras: I need re-assignment, Tom. I want out of this job. It's wrong. It's no good.
Tom, President of University: You're the best we've got.
Father Damien Karras: Am I 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. I think I've lost my faith, Tom.
[after fighting with Karl]
Burke Dennings: So, what's for dessert?
[to a prominent senator at Chris' party]
Burke Dennings: There seems to be an alien pubic hair in my gin. Never seen it before in my life! Have you?
Sharon Spencer: I should have known better. I'm sorry.
Chris MacNeil: Yeah, I guess you should have.
Sharon Spencer: How were the tests?
Chris MacNeil: We have to start looking for a shrink.
Chris MacNeil: How do you go about getting an exorcism?
Father Damien Karras: I Beg your pardon?
Chris MacNeil: Oh no, that was no spasm. 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.
Lt. Kinderman: If certain British doctors never asked "What is this fungus?" we wouldn't today have penicillin, correct?
Chris MacNeil: How does a doctor end up as a priest?
Father Damien Karras: It's the other way around, the Society put me through Medical School.
Father Damien Karras: [praying over Merrin's body] Ego te absolvo in nomine Patris, et Filiii, et Spiritus Sancti. Amen.
Demon: [as Father Merrin enters the house]
[with terrible rage]
Demon: MERRIN!
Karl: It wants no straps.
Demon: Give us time... Let the girl die... I am no one... I am no one... Fear the priest... Fear the priest... Merrin... Merrin.
Father Merrin: [looking at the Pazuzu Amulet] Evil against evil.
Father Dyer: Listen, if you ever go up there again will you take me along?
Astronaut: What for?
Father Dyer: First missionary on Mars.
[last lines - 2000 version]
Father Dyer: You know, you look a bit like Bogart.
Lt. Kinderman: You noticed.
Dr. Taney: 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 MacNeil: So what's wrong with her?
Dr. Klein: We still think the temporal lobe.
Chris MacNeil: Oh what are you talking about, for Chrissakes? Did you see her or not? She's acting like she's fucking out of her mind, psychotic, like a... split personality or...
Dr. Taney: 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 MacNeil: So, what's next?
Dr. Taney: A pneumoencephalogram, I would think. Pin down that lesion. It will involve another spinal.
Chris MacNeil: Oh, Christ.
Dr. Taney: What we missed in the EEG and the arteriograms could conceivably turn up there. At least, it would eliminate certain other possibilities.
Burke Dennings: Shall we summon the writer? He's in Paris.
Chris MacNeil: Hiding?
Burke Dennings: Fucking.
Lt. Kinderman: You know who you look like? John Garfield. Exactly, John Garfield, "Body and Soul." Do people ever tell you that, Father?
Father Damien Karras: Do people tell you that you look like Paul Newman?
Lt. Kinderman: Always.
Tom, President of University: [discussing the case, referring to Karras] Well, he does know the background. I doubt there's any danger in just having him assist. There should be a psychiatrist present, anyway.
Bishop Michael: What about the exorcist? Have you any ideas?
Tom, President of University: How about Lankester Merrin?
Bishop Michael: Merrin? Why, I'd a notion he was over in Iraq. I think I read he was working on a dig near Nineveh.
Tom, President of University: Yeah, you're right, Mike, but he's finished. He came back three or four months ago. He's at Woodstock now.
Bishop Michael: What's he doing there? Teaching?
Tom, President of University: No, he's working on another book.
Bishop Michael: Don't you think he's too old, Tom? How's his health?
Tom, President of University: He must be all right, he's still running around digging up tombs. Besides, he's had experience.
Bishop Michael: I didn't know that.
Tom, President of University: Ten, twelve years ago, I think, in Africa. The exorcism supposedly lasted several months. I heard it damn near killed him.
Chris MacNeil: Is it over?
Father Damien Karras: No
Chris MacNeil: Is she going to die?
Father Damien Karras: No!
Regan MacNeil: I heard... differently.
Chris MacNeil: Would you like some bourbon in that, father?
Father Merrin: Well, my doctor says I shouldn't but thank God my will is weak.
Lt. Kinderman: [to Chris] A draft in the fall when the house is hot is a magic carpet for germs.
Father Damien Karras: There isn't a day in my life when I haven't felt like a fraud. I mean priests, doctors, I've talked to them all. I don't know anyone who hasn't felt that.