A mysterious and extraordinary mental patient, named Prot, convinces both staff and fellow patients that he comes from a utopian planet called K-PAX. While his psychiatrist, Dr. Mark Powell, is baffled by this cheerfully confident man, Prot's description of life on a planet 1000 light years away awakens a sense of possibility among the other patients. The doctor gradually begins to witness this so-called alien having a remarkable effect on the mental health of the hospital's other patients. When Prot says he's going back before the summer ends, all the patients in the ward vie to go with him. As the days before Prot's announced departure from earth race by, Dr. Powell struggles with questions of science. Determined to prove the stranger is nothing more than a tragic victim of multiple personality disorder, the doctor soon finds himself doubting his own diagnosis.