The mysterious character Prot, portrayed by Kevin Spacey, who early on finds himself locked up in a mental hospital in Manhattan, claims to be from a distant planet one thousand light years away called K-PAX. Dr. Mark Powell, Portrayed by Jeff Bridges, is the psychiatrist at this mental institution that examines Prot and tries to figure out how.
K-PAXian is the term used in the books to refer to the inhabitants of the planet K-PAX. Their language is pax-o. In the book, all knowledge of K-PAX and its attendant species comes from the character prot.