The Book of Mormon is purported to be a record of the ancient inhabitants of the Americas, which were uninhabited until the migration of a family of Jews who, according to the record, were instructed by divine revelation to construct ships according to divinely inspired plans to sail west to escape the conquest of Babylon. The family stayed below in the ship or ships (it's been a while since I read the thing) and God guided the vessel(s).
Upon landing upon the shores they received instructions from God and began to multiply. Two major nations sprang from the first family, the Lamanites and the Nephites, named for two sons of Lehi, the patriarch of the family, Laman and Nephi. The Lamanites were wicked, and God cursed them with dark skin. The native Americans are the remnants of the Lamanites. The Nephites were righteous for longer and remained white.
After Christ's Ascension in the Eastern Hemisphere, He descended upon the Americas and began his earthly ministry all over again. He came to the whites, and chose the twelve most righteous among them to be his Apostles, and they traveled around preaching mostly to the Nephites. There was no second crucifixion, but Christ left them by ascending from a mountain top. After a while, the Nephites became very wicked, but instead of changing their skin, God allowed them to be completely exterminated by the Lamanites, with the exception of three who were spared physical death, and still, from time to time, are reported to appear to the faithful.
From the time of their arrival until the extermination of the Nephites, the faithful had been keeping a book and adding to it from time to time until it's final redaction by Mormon, who was also one of its last contributors. The book was hidden in what is now New York to keep it safe from the corruptions of the Lamanites, until it was found and translated by Joseph Smith.