A man calling himself simply Prot (Kevin Spacey) is picked up in grand central station in New York City after mysteriously appearing. When he claims to be an alien from the planet K-Pax he is quickly whisked off to a psychiatric hospital and in to the hands of Dr. Mark Powell (Jeff Bridges). During his sojourn in the psychiatric hospital Dr. Powell repeatedly tries to determine the indentity of this mysterious man Prot. Even as Dr. Powell's attempts to discover Prot's true identity continue, Prot has an astonishing impact on the other patients in the ward. Prot's continuing assertions of his alien origin truly frustrate Dr. Powell and he becomes even more determined to figure him out. Is he really from K-Pax, and is he really Prot, these are the questions that Dr. Powell is determined to answer.