After being mugged, Prot (Spacey) is sent to a mental institution when he mentions to the cops that the light on Earth is far brighter than on his own planet K-PAX. Prot soon meets seasoned psychiatrist Dr. Mark Powell (Bridges) who figures Prot has delusions about his career, and it will only be a matter of time before he will return to reality.
Prot explains he is on a fact-finding mission and that he will be going home before the summer ends, exciting the other mental patients. Meanwhile, Dr. Powell uses every tool at his disposal to discover Prot's truth, but as those tools fail him time and time again, Powell begins to doubt things he has accepted his entire life.
When Prot provides information that confounds astronomers, Powell starts to believe that Prot really is from outer space.
Based on the novel by Gene Brewer.