Two elderly aliens looking for their grandkids crash-land on Earth.

When he was 12 years old, Bill Cody, later knew as Buffalo Bill, is rider for "pony express" carrying the mail through the wilds of America. It becomes later caravans guide. When driving one of them meets Luisa, the niece of a priest who tried to evangelize the savage tribes accompanied by a converted Indian. It is a dangerous time because, before the advancing white man, the Sioux tribes are buying weapons from unscrupulous dealers...