Bill Bixby, as newspaper reporter Tim O'Hara, stumbles across a crashed spaceship. He finds a Martian, played by Ray Walston, who he befriends and protects while the alien repairs his ship.
Disguised as a 70-year-old, Martin becomes a night watchman.
The Earthly environment is causing a vitamin deficiency in Martin. Martin needs some vitamins immediately or else he will grow weaker and weaker to the point of no return.