Two-time Academy Award winner Kevin Spacey portrays an innocuous man detained at Grand Central Station in the aftermath of a mugging. Multiple Academy Award nominee Jeff Bridges plays a seasoned psychiatrist at an under-staffed public hospital.
The police turn Prot (Spacey) over to Dr Mark Powell (Bridges) after he politely tells them that the light on their planet is too bright for him - much brighter than back home on the distant planet K-PAX.
Prot says he's on a fact-finding mission, and patiently explains how his experiences on earth contrast with life on K-PAX. He finds much to like here, but says he will go home before the summer ends. As the days before his announced departure race by, all the patients on the ward vie to go with him.
Dr Powell is haunted by dark fears for Prot, and is determined to use every tool at his disposal to discover this man's truth before it's too late. But as those tools fail him, Powell begins to doubt things he has accepted as true his entire life. It gets worse when Prot's claims are put to a group of skeptical astronomers who are confounded by the things Prot seems to know.
Powell has to ask himself if Prot could somehow be the real thing. And as he continues his efforts to penetrate Prot's shell, he gradually sees how impenetrable he has allowed his own veneer to become.