This film tells the story of the Mormon exodus to the west in the face of unremitting persecution, seen through the eyes of a participant (Tyrone Power). Dean Jagger plays a good Brigham Young, if perhaps a little less forceful than the real one, while Vincent Price is an excellent Joseph Smith.
Hollywood being Hollywood, there are places where dramatic effect is allowed to take precedence over historical accuracy. In particular, the murder of Joseph Smith is shown as coming after a jury has been terrorised by the mob into finding him guilty, when in fact it came before any trial could take place, so that the entire courtroom scene is fictitious. This, of course, was precisely why Joseph's enemies lynched him rather than permitting the law to take its course. They knew they could not convict him of anything serious enough to end his career.
Polygamy is not altogether ignored, but on the whole is rather played down, with only one of Brigham's wives appearing on screen, though the existence of others is mentioned. Presumably, going further than that would have been inappropriate for Hollywood family entertainment.
Later, Brigham Young is shown as leading the Saints across the frozen Mississippi (true thus far) just one jump ahead of the mob, and they all look back to see Nauvoo burning behind them. In fact, the evacuation took place over more than six months, with Brigham leading the first party out in February 1846, and the last not going until September. As far as I know, the City of Nauvoo was never burned (though the Temple was) but just gradually looted after its abandonment. It is however true that the Saints remained subject to vicious attacks long after they were clearly in process of leaving, and posed no possible threat or danger to anyone.
The hardships of the Pioneer Trail are well portrayed, and, perhaps in a sign of changing attitudes, the Indians are shown in an entirely favourable light. The film ends on a really dramatic note, when the Saints are fighting desperately to save their first harvest from the crickets, and are saved at the last minute as the crickets in turn are eaten by seagulls. I understand that the cast had to perform this scene for real, there being no way to fake it with the special effects available in 1940, and found it every bit as unpleasant as it probably was first time round.
All in all, an excellent movie portrayal of one of the greatest pioneer epics of the American West. Shame it's only available in Region 1, as it deserves a wider audience. Still, multi region players aren't too expensive these days. Enjoy it.