Their "son" is actually a man in his early 20s named Frederic Bourdin, that has a history of taking on the identities of lost children all around the world. A family member flies to Spain and somehow believes that the dark-skinned, brown-eyed Frenchmen is actually their long lost blonde haired, blue-eyed relative.
They travel back to Texas and somehow, everyone, including government workers, is fooled into believing Frederic is really Nicholas. Everyone except a gray-haired, country private investigator who believes it's "all in the ears."
The movie is more of a documentary type flick. Frederic, Nicholas' family and other people dealing with the situation are interviewed and the crazy story is told. It's a bit messed up how some people in the movie are and there's plenty of head scratching movements, but it was an entertaining watch.