In 1869, ten men in four boats set out to explore the last unknown territory in America. A one-armed, Civil War major, John Wesley Powell - whom one of the world's most beautiful, scenic lakes has since been named after - leads the first river adventure down the Colorado River, through the Grand Canyon. He is determined to make his mark as a scientist and an explorer, and will risk the lives of nine men to do it. Three months later, only six men and two boats emerge on the other side. Their epic journey begins as a scientific quest, and ends as a struggle for survival.