Begins on July 28, 1805, more than one year after Meriwether Lewis and William Clark set out on their epic journey. The expedition now enters upon perhaps the most difficult part of its route, from the Three Forks of the Missouri River in present-day Montana, over the Bitterroot Mountains, and to the Cascades of the Columbia River on today's Washington-Oregon border. The explorers encounter Shoshone, Flathead, Nez Perce, and other Indian tribes, some of whom have never before met white people.