A person appears "out of no where" at the NY Central station. He helps a woman being mugged and is arrested by the police, when he gives weird answers to their questions.After claiming he is an extraterrestrial from the planet 'K-PAX', 1,000 light years away in the Lyra constellation, Prot (Kevin Spacey) (not capitalized and pronounced with a long O, rhyming with boat) is committed to the Psychiatric Institute of Manhattan. There, psychiatrist Dr. Mark Powell (Jeff Bridges) attempts to cure him of his apparent delusions. However, Prot is unwavering in his ability to provide cogent answers to questions about himself, K-PAX, and its civilizations. His medical examination only reinforces his story, as Prot can see ultraviolet light and he is completely resistant to the effects of Thorazine. Powell introduces him to a group of astrophysicists (Dr. Chakraborty (Ajay Naidu), Dr. Steven Becker (Brian Howe) & Dr. Claudia Villars (Alfre Woodard)) who are befuddled when Prot displays a level of knowledge about his claimed star system that was unknown to them. Prot claims to be 337 yrs old, arrived on Earth 4 yrs and 9 months ago and aged by 7 yrs while travelling to Earth at 6 times the speed of light. There are no families on K-PAX and reproduction is very unpleasant. No laws, no lawyers and no punishments for crimes.Mark is married to Rachel Powell (Mary McCormack), who feels that Mark is neglecting the family. Mark is divorced from his first wife and has a son Michael Powell (Aaron Paul), who is in college, but wont speak to Mark.Prot also wins over the other patients at the Institute (Ernie (Saul Williams) who is bacteria phobic, Doris Archer (Celia Weston) who went mad after she her partner abandoned her and she has been waiting for 11 yrs, Sal (Peter Gerety) & Howie (David Patrick Kelly)), each of whom believes unquestioningly that he is indeed from K-PAX. Prot, who claims to have journeyed to Earth by means of "light-travel" (and his beings can travel at several times faster than the speed of light) (He says Earth is an early stage evolution planet with an uncertain future), explains that he can take one person with him when he returns. Thereafter, most of the patients at the Institute ask Prot to take them with him. Prot tells Howie to complete 3 tasks in order to be cured. The first task is to find a blue bird and Howie gets very excited when he finally views one through the window (an extremely rare occurrence). The 2nd task was to almost strangle Ernie to death. Ernie realizes that death is something that he has no control over, and is cured of his germ-phobia. Last task was to stay on Earth and be prepared for anything.Upon learning that many of his patients expect to leave Earth on July 27 (at 5 51 am eastern time, as entries and exits from K-Pax have to be coordinated, just like an ATC), Powell confronts Prot, who explains that it is a predetermined date. However, Powell believes this to be a significant date in Prot's life, a day on which he suffered a severe psychological trauma. Prot demonstrates that he can speak to animals as well. Powell decides to subject Prot to regression hypnosis (mark invites Prot to his home for 4th of July. Everything was going well until Mark's kids turned on the sprinkler and ran into it, and that's when Prot lost his cool and tries to stop Mark's daughter from running into the sprinkler and has to be subdued), which works well. Using information gained from these sessions, Powell figures out that Prot may simply be an alter ego of Robert Porter, a man from New Mexico who attempted suicide in 1996 after his wife and child were murdered. Prot claims that Robert could contact him, and had done so several times in the past. and that day, when his wife and child was murdered, Robert called Prot to Earth to help him. The local sheriff tells Mark that Robert came home from work and found his wife and daughter murdered and proceeded to kill the murderer by snapping his neck like a twig. Powell tries to confront Prot with this knowledge, but Prot's reaction is one of bemusement, and he cryptically tells Powell that he hopes he will take good care of Robert now that he has found him. Prot also tells Mark to spend more time with his family and to reconnect with Michael.On July 27 as the hospital staff watch, the camera in Prot's room cuts to static at the precise time Prot said he would leave Earth. Powell finds Porter lying on the floor in his room, catatonic, Prot having apparently left Porter's body for the light travel back to K-PAX. The other patients do not recognize Robert, as he is being wheeled out of the room. In addition, one of them is missing: Bess (Melanee Murray), a woman who had remained mute since her home was destroyed in a fire and who had been among the patients that asked to go to K-PAX with Prot. She is never found. Powell continues to take care of the catatonic Porter and tells him about how the patients he helped have gone on to live normal lives again (Howie got a job at the public library, Ernie is a crisis counselor), but Robert does not respond. Powell is left with no absolute answer as to whether Prot was in fact an alien entity or just a coping mechanism of the traumatized Porter, but seems far from convinced that Porter's behavior was a delusion.In a final voiceover, Prot explains to Powell that the people of K-PAX have discovered that our universe will repeat its events again and again, so the mistakes we make will be repeated forever. Prot encourages Powell to make this time count, as it is the only chance we have. Inspired, Powell begins a new, better life by reconciling with his estranged son.