PROT is a patient at a mental hospital who claims to be from a far away Planet. His psychiatrist tries to help him, only to begin to doubt his own explanations.

Prot: I wanna tell you something Mark, something you do not yet know, that we K-PAXians have been around long enough to have discovered. The universe will expand, then it will collapse back on itself, then will expand again. It will repeat this process forever. What you don't you know is that when the universe expands again, everything will be as it is now. Whatever mistakes you make this time around, you will live through on your next pass. Every mistake you make, you will live through again, & again, forever. So my advice to you is to get it right this time around. Because this time is all you have.
Dr. Mark Powell: How do you know right from wrong?
Prot: Every being in the universe knows right from wrong, Mark.
Prot: Let me tell you something, Mark. You humans, most of you, subscribe to this policy of an eye for an eye, a life for a life, which is known throughout the universe for its... stupidity. Even your Buddha and your Christ had quite a different vision, but nobody's paid much attention to them, not even the Buddhists or the Christians. You humans. Sometimes its hard to imagine how you've made it this far.
Ernie: Dying is something you have no control over. Why waste your life being afraid of it?
Dr. Mark Powell: What if I were to tell you that according to a man who lived on our planet, named Einstein, that nothing can travel faster than the speed of light?
Prot: I would say that you misread Einstein, Dr. Powell. May I call you Mark? You see Mark, what Einstein actually said was that nothing can accelerate to the speed of light because its mass would become infinite. Einstein said nothing about entities already traveling at the speed of light or faster.
Howie: You never gave me my third task. What's my third task?
Prot: To stay here, and be prepared for anything.
Prot: Doctor. Doctor. Doctor. Doctor. How many doctors are there on this planet?
Prot: Doctor... patient... curious distinction.
Dr. Mark Powell: Uh, how is it that being a visitor from space, that you, uh, you look so much like me or, or anyone else from Earth?
Prot: Why is a soap bubble round?
Dr. Mark Powell: "Why is a soap bubble round?"
Prot: You know, for an educated person, Mark, you repeat things quite a bit. Are you aware of that? A soap bubble is round because it is the most energy-efficient configuration. Similarly, on your planet I look like you. On K-PAX I look like a K-PAXian.
Dr. Mark Powell: Prot, why did you want to come to our planet?
Prot: Well, I've been here many times before. But what brought me here first? I don't know. Pure curiosity, I guess. I'd never been to a Class BA-3 planet before.
Dr. Mark Powell: Class BA-3?
Prot: Early stage of evolution. Future uncertain.
Prot: [eating a banana] Your produce alone has been worth the trip.
Dr. Mark Powell: Could you tell me a bit about your boyhood on K-PAX? Where were you born? You were, uh, born, right? K-Paxians have babies?
Prot: Oh, yes. Much like on Earth, but unlike you humans, the reproductive process is, uh, quite unpleasant for us.
Dr. Mark Powell: Could you compare the effect to something that I might understand. Like a toothache...?
Prot: It's more like having your nuts in a vise, except we feel it all over. And to make matters worse, the sensation is associated with something like your nausea accompanied by a very bad smell. The moment of climax is like being kicked in the stomach and then falling into a pool of mot droppings.
Dr. Mark Powell: Mot droppings?
Prot: A mot is a being much like your skunk only far more potent.
Dr. Mark Powell: Uh, if it's such a terrible experience, uh, how do you reproduce?
Prot: As carefully as possible.
Prot: Don't get me wrong, Mark. You've been very hospitable.
[pause, Prot looks around]
Prot: Hospital. Hospitable.
[Prot smiles]
Prot: You know what I've learned about your planet? There's enough life on Earth to fill 50 planets. Plants, animals, people, fungi, viruses, all jostling to find their place, bouncing off each other, feeding off each other. Connected.
Dr. Mark Powell: You don't have that kind of connection on K-PAX?
Prot: Nobody wants, nobody needs. On K-PAX, when I'm gone, nobody misses me. There would be no reason to. And yet I sense that when I leave here... I will be missed. Yes. Strange feeling.
Dr. Mark Powell: Oh, 6:15 was late. Didn't leave the station till 6:30. Should have caught the nearest beam of light.
Dr. Chakraborty: He most definitely has a sensitivity to white light, but I think it's his range that you'll find interesting.
Dr. Mark Powell: What am I looking at?
Dr. Chakraborty: Uh, he can detect light at a wavelength of up to 300 to 400 angstroms. Ultraviolet. Of course, Prot's explanation is that due to his planet's peculiar quality of light caused by its two suns, K-PAXians are used to light conditions much like our twilight most of the time.
Dr. Mark Powell: Whoa, w-wait, wait a minute. Uh, Chuck, I didn't think human beings could see ultraviolet light.
Dr. Chakraborty: We can't.
Prot: [after "talking" to the dog] She says she doesn't like it when you hide her favorite tennis shoe, and she doesn't hear so well on her left side, so don't sneak up on her anymore.
Claudia Villars: [after prot has disappeared] Patients do not escape from this institution. They don't escape. I'm going to have a great time explaining this to the state board. I've got psychotics on the fourth floor packing up their sneakers because they all think they're going off to K-PAX. Find him.
Dr. Mark Powell: I'd like to begin by asking you... if you know why you're here.
Prot: Of course. You think I'm crazy.
Dr. Mark Powell: I prefer the term "ill". Do you think you are... ill?
Prot: A little homesick, perhaps.
Dr. Mark Powell: Really? Where is home?
Prot: K-PAX.
Dr. Mark Powell: K-PAX?
Prot: Capital "K," hyphen, capitals "P-A-X." K-PAX is a planet. But don't worry. I'm not going to leap out of your chest.
Dr. Mark Powell: I'm not worried. It's just that I'm only familiar with nine planets.
Prot: Well, actually, there are ten, but that doesn't matter. I'm not from your solar system. K-PAX is about 1,000 of your light-years away from here in what you would call your constellation Lyra.
Prot: I will admit the possibility that I am Robert Porter, if you will admit the possibility that I am from K-PAX. Now if you'll excuse me, I have a beam of light to catch.
Prot: [after Mark gives him his pen] A much more efficient writing tool.
Prot: I travel light.
Dr. Mark Powell: Have a seat.
Prot: "Have a seat." What a curious expression.
[On why he always wears sunglasses]
Prot: Your planet is really bright.
Prot: I shall miss Earth, it has great potential.
Dr. Mark Powell: Well, let's hope extraterrestrials qualify for Medicaid.
Prot: Do I smell apple pie?
Dr. Mark Powell: You know, maybe what's wrong with him is that he is.
Claudia Villars: Is what?
Dr. Mark Powell: From the planet K-PAX.
Rachel Powell: [notices husband not paying attention] "My head fell off this morning; I sewed it back on with dental floss... waxed, of course"
Prot: We don't have families on K-PAX.
Rachel Powell: Well, you don't know what you're missing.
Prot: You seem overly upset, Mark. To borrow a phrase from Navarro, you need to chill.
Dr. Mark Powell: What would you say if I told you I think you're as human as I am?
Prot: I would say you're in need of a thorazine drip, Doctor.
Dr. Mark Powell: What happened?
Ernie: My good friend Howie... he, he just about strangled me to death.
Dr. Mark Powell: What?
Ernie: Son of a bitch I love him!