PROT is a patient at a mental hospital who claims to be from a faraway planet named K-PAX. His psychiatrist tries to help him, only to begin to doubt his own explanations.

A story of a mysterious stranger who defies convention, puzzles the experts and leaves everyone searching for the truth. A stranger who calls himself Prot seemingly appears out of nowhere following a disturbance at New York's Grand Central Station.

Claiming to come from the distant planet K-PAX, Prot draws the attention of jaded Dr. Powell, whose initial skepticism soon turns to fascination and amazement. Even a team of leading scientists are at a loss to explain Prot's detailed knowledge of the star system he calls home. But with Prot's return to K-PAX fast approaching, the search for answers intensifies and the mystery takes on pulse-quickening proportions.