This is a compelling, well-researched work of historical fiction. The linchpin of the story is John D. Lee, a close personal friend of Brigham Young and a charismatic Mormon leader, who was tried and convicted  twenty years after the fact for the 1857 Mountain Meadows Massacre, in which over a hundred, non-Mormon immigrants to the West were slaughtered by Mormon settlers with the possible help of their Native-American allies.