After being arrested in the Grand Station Central of New York City, a man calling himself Prot (Spacey), who claims to have travelled to Earth from his home planet of K-PAX, is taken to the Manhattan Psychiatric Centre because he is believed to be mentally ill. Mark Powell (Bridges), a psychologist, meets with the man, named Prot, and decides to treat him. Prot seems peaceful and is extremely knowledgeable, even baffling astrophysicists with his uncanny knowledge of astronomy. However, when Prot has a seemingly violent episode at Dr Powell's house, Dr Powell proceeds with regression therapy to find out more about Prot's past. The therapy leads Dr Powell to discover an emotionally painful history, linked to a man named Robert Porter. Time is running out, as Prot says he will be leaving in a few days, and Dr Powell tries to help get to the truth. Meanwhile, Prot meets the other patients of the Psychiatric Centre and takes it upon himself to help them, in his own way. Is Prot really from outer space or is he the product of a deluded mind?