Navin R. Johnson (Steve Martin) is a dim-witted White man who was adopted as an infant by a poor rural southern Black family. Never able to follow the beat of Rhythm and Blues or Gospel, one day as a young man he hears a Lawrence Welk song on the radio and able to follow the beat, he realizes there is more to the world than the 2 room shack they live in, and he decides to go out into the world to seek his fortune.He hitch-hikes to St. Louis where an overtly Jewish gas station owner, Mr. Hartounian (Jackie Mason), gives him a job and a back room to stay in. Navin starts sending a little money home to his adoptive family whenever he can. While pumping gas, Navin comes up with a fix for a fast-talking salesman customer whose eyeglasses keep slipping off his nose. It is a wire loop with a nose brace. The grateful man thinks he can market the invention and promises Navin 50% of any profits. Navin gets into the local phone book, from which a crazed gunowner (M. Emmet Walsh) randomly picks him as a target for assassination. Navin is shot at, but manages to flee, and winds up in a carnival trailer heading out of town.Navin gets a job as a weight-guesser with the carnival, and loses his virginity to a stunt-daredevil woman. He meets and falls in love with a carnival goer, Marie (Bernadette Peters), a cosmetologist. They get married on the spur of the moment by the first legally authorized person they can find to perform the ceremony, who happens to be a Black Witch Doctor.The salesman tracks Navin down with a Private Eye who happens to be the crazed assassin gone straight. Navin's invention has been a huge success and Navin starts getting checks for large sums of money. He hires servants and then purchases an extravagant mansion. Just as it look as if life cannot get any better, he is sued by purchasers of the "Opti-Grab" device. Apparently, it makes wearers cross-eyed. Navin loses all his money, and wanders off to become a homeless bum. Marie dumps him and he is heartbroken.At the film's end, his Black family and Marie find him on skid row. Apparently his foster-father had invested the relatively modest sums of money Navin sent home from his various jobs, and the family is much better off. In the closing scene, we see that they have built a much-larger unpainted shack to replace the old one.