Mormon History Movie Review: Glad Tidings: The History of Baptisms for the Dead

http://www.lds.org/media-library/video/church-history/02-church-history?lang=eng

When Joseph's brother Alvin died, the family was distraught.  Their grief was only more pronounced when the preacher who performed the funeral told the family that Alvin was condemned because he had not been baptized.
Later, in the Kirtand temple, Joseph received a vision of the Celestial Kingdom, and was surprised that Alvin was there.  The Lord explained that those who would have accepted the gospel, had they been given the opportunity, may inherit all of the blessings of heaven. 
It was while the saints were in Nauvoo that Joseph received the revelation for performing baptism for the dead.  These were originally performed in the Mississippi river.  Later Joseph received added direction that these should be performed in the temple.