In the spring of 1804, at the behest of President Thomas Jefferson, a party of explorers called the Corps of Discovery crossed the Mississippi River and started up the Missouri, heading west into the newly acquired Louisiana Territory.

The expedition, led by two remarkable and utterly different commanders - the brilliant but troubled Meriwether Lewis and his trustworthy, gregarious friend William Clark - was to be the United States' first exploration into unknown spaces.

The unlikely crew came from every corner of the young nation: soldiers from New Hampshire and Pennsylvania and Kentucky, French Canadian boatmen, several sons of white fathers and Indian mothers, a slave named York, and eventually a Shoshone Indian woman, Sacagawea, who brought along her infant son.

Plentiful excerpts from the journals kept by the two captains and four enlisted men convey the raw emotions, turbulent spirits, and constant surprises of the explorers, who each day confronted the unknown with fresh eyes. An elegant preface by Ken Burns, as well as contributions from Stephen E. Ambrose, William Least Heat-Moon, and Erica Funkhouser, enlarge upon important threads in Duncan's narrative, demonstrating the continued potency of events that took place almost two centuries ago.

And a wealth of paintings, photographs, journal sketches, maps, and film images from the PBS documentary lends this historic, nation-redefining milestone a vibrancy and immediacy to which no American will be immune.