Though Andrew Henry is a successful businessman, he has little experience in the fur trade when he joins a trapping expedition headed into the uncharted distant West. His partner, a mercurial Spanish-American trader named Manuel Lisa, nearly starts a mutiny by antagonizing American trappers while favoring the more experienced Canadian rivermen. When Lisa suddenly departs the expedition, Henry takes charge and vows to find the lucrative beaver furs in a region called Three Forks, which is ruled by the fearsome Blackfeet tribe. Along the way, he enlists the help of famous mountain man John Colter, whose dire warnings about Three Forks cause deep unease among the men. Colter's fears come true when the Blackfeet strike again and again, killing some of the men and forcing the rest to take refuge in a hastily built fort. Before long, Henry must make an agonizing choice: break his word and abandon the trapping venture, or lead the remaining men deeper into darkness to complete his mission.