A mysterious psychiatric patient named Prot claims to everyone in the hiospital he's an alien from a 1000-light-year-from-Earth planet called K-PAX. During his stay, Prot's demonstration of life on K-PAX is really an ultimate inspiration for his fellow patients, and especially for his psychiatrist, Dr. Powell. As Prot's announced departure from Earth to his homeland is fast approaching, Dr. Powell finds himself struggle with questions of science, and start doubting his own diagnosis.