The Corps of Discovery's Permanent Party makes it back to St. Louis in this episode, finishing the grand march across a continent and back on which it set out two years prior. This is the tale of how the Lewis and Clark Expedition made it back from Fort Clatsop to St. Louis in one travel season. It's the story of fighting up rivers, marching through the Bitterroot Mountains in a heavy season of snow, splitting the party up despite the immense risks so that the Marias River and Yellowstone River Valley could be explored as the party filled in the map and looked for commercial opportunity, and how the party split with Sacagawea when they returned to the Mandans.

This is also the story of Pompey's Tower, of a desperate fight with the Blackfeet Indians, of the discovery that the Northwest Passage was truly non-existent, of the wisdom of the Nez Perce Indians, of the first mountain man—a veteran of the Permanent Party, and of the tragic fate Lewis met with after an immensely successful journey