k-pax (2001)

a poor man, very traumatized but endowed with a shocking knowledge of astronomy and the implications of human weaknesses, quietly declares to be an alien on a mission from the planet k-pax and for this he is thought crazy and locked up in a mental hospital. after spending a few days in prison, he ends up under the care of a psychiatrist who is captivated by the qualities of his new patient and begins to take the personality of the strange individual very seriously.