By 1869, most of the USA had already been mapped and charted with the exception of the Colorado River region. Therefore, the U.S. government sends geologist and ex-Union Army Major John Wesley Powell to map it. Major Powell, who's missing a hand since the Civil War, needs assistants but trustworthy skilled men are hard to find after the war. The Major has to accept any volunteer he can find and he assembles a team of nine men. The team departs from Green River, Wyoming, in four rowboats and is made up of men of different ages, backgrounds, personalities and motivations for joining up. In one rowboat there are two former army officers who fought on opposing sides during the Civil War. In another rowboat, one man is a recovering alcoholic and the other is a boozing Englishman who constantly taunts the abstinent. Second-in-command is Major Powell's brother, Walter, whose abrasive personality constantly offends the team. The Major excuses his brother's frequent violent outbursts by explaining that Walter's psyche was damaged during the war in a POW camp. Trapper and frontier scout William Dunn joined the team to be away from civilization. Billy 'Missouri' Hawkins joined to cook for the team but some men suspect him of being on the lam. Brothers Oramel and Seneca Howland complete the team. At first, the men are enthusiastic and their spirits are high but as the voyage unfolds their conflicting personalities are leading to frequent clashes. Moreover, the team has to deal with treacherous river rapids, bad weather, water and food shortages, unforeseen accidents, rugged river banks, poisonous rattlers and murderous Ute Indians. Also, a presumed gigantic waterfalls awaits at the river's end for any unwise traveler foolish enough to venture down the Colorado river.