Under direct orders from President Thomas Jefferson, Meriwether Lewis and William Clark lead the first expedition of its kind through the uncharted frontier. In hopes of reaching the Pacific Ocean, Lewis and Clark's team travels by boat up the Missouri River, but the waters become too shallow to cross the Rocky Mountains. With no way to travel by land with their supplies, Lewis and Clark rely on a young Shoshone woman named Sacagawea who is accompanying them on the expedition to communicate with the Shoshone tribe and set up a trade for the horses that they need to cross the Rocky Mountains before winter and successfully reach the Pacific Ocean.