Overview
"The Chameleon" is an episode of the original The Outer Limits television show. It first aired on 27 April 1964, during the first season.
Opening narration
"The race of Man is known for its mutability. We can change our moods, our faces, our lives to suit whatever situation confronts us. Adapt and survive. Even among the most changeable of living things, Man is quicksilver-more chameleon-like than the chameleon, determined to survive, no matter what the cost to others... or to himself."
Plot
A flying saucer has landed in a remote part of the United States and wiped out a military patrol sent to investigate. Falsely believing that the saucer contains nuclear material, the authorities decide on a wild scheme: they recruit Louis Mace, a disaffected CIA agent, to infiltrate the ship.
Genetically modified to pass as an alien, Mace finds that he has unique insight into the alien "invaders" nature, the genetic material that has been used to modify his appearance seemingly taking precedence over his human nature. He also begins to question his allegiance, and eventually sides with the aliens, finding out that they're benevolent and have no desire to come in contact with humans in any way, being simply stranded on Earth after their ship's engine broke down.
From Wikipedia (CC BY-SA 4.0).