In the 1840s, a newspaper reports that a party traveling west to California on an unconventional and particularly difficult route set by a man named Jefferson Clay gets lost in the mountains during a winter storm. One of the travelers, John Buell, documents their progress across the American West beginning at their outset in Missouri. At first, travel is difficult but well-paced, and the party is in good spirits despite his mother-in-law’s death of illness. However, as they get farther west to forts along the route, men begin to argue with Clay that his chosen path is far too difficult for travel. The leader ignores them and the group continues on.  As they deviate from the normal route, the troubles begin. The group gets lost in canyons and set back days, and a man named Bowden says they should stand up to Clay and turn back. John cautions patience, and they wait. The travel across a salt desert proves to be even worse, and the group loses time, morale, deserters, and many wagons. Clay and Bowden get into an altercation that ends with both of them dead, and the group continues on to cross the Sierra Mountains, which are now capped with snow. Within days, they are trapped at a lakeside outpost of cabins by a winter storm. John’s wife Elizabeth takes over his diary to report what events have transpired since the party was trapped in the mountains. As they ran out of food, John left with a few other men to seek out help. Another man named Hiderick argued that the remaining people had to eat human flesh to survive, and eventually the other desperate travelers give in, although Elizabeth and her child Mary-Kate refuse even when the scouts return with John’s dead body. More and more settlers die around them until they are left alone with Hiderick, who puts on the skin of John’s face and threatens to come for them. Elizabeth shoots him, but knows they don’t have much time. She shapeshifts into a wolf, which her mother had always hinted was in their blood, and kills Hiderick, but realizes then that she can’t shift back. She protects Mary-Kate until a rescue party finally comes up the mountain to witness the horror of the travelers’ slaughter, then allows them to take her daughter to safety in California while she stays with her new pack.