On May 14, 1804, Meriwether Lewis, William Clark, and the Corps of Discovery headed up the Missouri to explore the vast unknown territory just purchased by the United States from France. Sent by President Thomas Jefferson, they sought geographic and scientific knowledge—and the fabled Northwest Passage, a water route to the Pacific. Accompanying the two famous captains were young army men; French-Canadian boatmen; Clark's African-American slave, York; and a Shoshone woman named Sacagawea who brought along her infant son. Hal Holbrook narrates.

With the help of guide Sacagawea and friendly Native American tribes, Lewis and Clark and the Corps of Discovery make it through the snowy Bitterroot Mountains. After crossing the Continental Divide, they find themselves at last traveling with the current instead of struggling against it. Finally they catch sight of their goal: the Pacific Ocean.