For the book see the article K-PAX.
K-PAX (2001) is a mystery and/or sci-fi drama about a mental patient who claims he is an alien. During his treatment, the patient/alien, prot ("rhymes with 'goat' and is not capitalized"), demonstrates an outlook on life that ultimately proves inspirational for his fellow patients and especially for his psychiatrist. The movie was based on a novel of the same name, itself similar to the 1986 Argentinian film Man Facing Southeast directed by Eliseo Subiela. It also bears striking similarities to the 1995 film Eyes Beyond Seeing in which an enigmatic mental patient played by Keith Hamilton Cobb claims to be Jesus.

Plot summary
The story opens with a man who calls himself "prot" (Kevin Spacey) who mysteriously appears in New York City's Grand Central Terminal. When he claims he is an alien from a planet called 'K-PAX', prot is handed over to a New York hospital psychiatric ward. A Manhattan psychiatrist, Mark Powell (played by Jeff Bridges), is given the task of treating and "curing" prot of his supposed delusions. Right from the first psychiatric session, Dr. Powell begins to discover prot's uncanny abilities and immense knowledge. As the story develops, Dr. Powell becomes quite attached to prot, and starts to believe his seemingly ridiculous claims. He even manages to baffle some of the foremost astrophysicists with his knowledge of astronomy.
Once prot displays violent behavior at Dr. Powell's house, the latter decides to regress him to find out more about prot's past. Because prot has promised he will leave Earth on July 27, it's a race against time for Dr. Powell to find out the real identity behind the enigmatic prot.
The film does not explicitly state who or what prot really is.