Miss Snoddy's strange dream about the Six Grandfathers' Mountain in the Black Hills launches Ava, Ellie, and Griffin into their next adventure. Wanting to know more about the sacred territory of the Lakota Sioux inspires them to join the Lewis and Clark Corps of Discovery Expedition. The friends are always eager to live history rather than simply study it. Captain Lewis gives Miss Snoddy and her students permission to come aboard his boat sailing up the Missouri River. The journey is going well until three girls disappear from the Mandan Indian village where the Corps crew are wintering. Determined to discover what happened to their new friends and bring them to safety, the kids form an unlikely alliance with Seamus, Captain Lewis' dog, and York, the African-American boyhood friend of Captain Clark. Aided by Sacagawea, herself kidnapped as a teen-ager, they must overcome the hardships of nature and the dangers of evil men if they are to successfully reunite with the Mandan girls.