Dr. Mark Powell (JEFF BRIDGES) is a psychiatrist at the Psychiatric Institute of Manhattan who's been treating various ill patients such as Howie (DAVID PATRICK KELLY), an obsessive compulsive; Ernie (SAUL WILLIAMS), who's fearful of germs; Sal (PETER GERETY), who believes everything stinks; and Mrs. Archer (CELIA WESTON), who hasn't come out of her room in a long time.
His latest patient, however, who goes by the name Prot (KEVIN SPACEY), is the most intriguing. Claiming to be from K-PAX, a distant planet some one thousand light years away, the otherwise human looking man doesn't seem to be affected by normal medication and reportedly can see otherwise invisible ultraviolet light.
Noticing that Prot seems to walk the walk and talk the talk of a convincing alien, Mark tries to figure out what would cause such an elaborate delusional state. As the doctor tries probing Prot for answers, he must put up with the patient's amused responses, as well as doubts from his associates, including Claudia Villars (ALFRE WOODARD), about his early diagnosis and plan for further investigation.
With the doctor and patient getting to know each other better from their various interchanges - including that regarding Mark's estranged son, Michael (AARON PAUL), from his first marriage -- Prot buoys the other patients' spirits by his announcement that he'll take one of them back to K-PAX with him upon his departure.
As Mark's continued detective work puts a strain on his marriage to wife Rachel (MARY McCORMACK) who cares for their kids Natalie (TESS McCARTHY) and Gabby (NATASHA DORFHUBER), he races against time and Prot's announced departure schedule to figure out if he's simply delusional or really is a visiting alien from another planet.