In 1832, Jim Deakins is travelling in the wilderness when he encounters an initially hostile Boone Caudill. However, they soon become good friends and head together to St. Louis on the Missouri River in search of Boone's uncle, Zeb Calloway. They find him when they are tossed in jail for brawling with fur traders of the Missouri River Company. When 'Frenchy' Jourdonnais comes to bail Zeb out, Zeb talks him into paying for Jim and Boone too. The two men join an expedition organised by Zeb and Frenchy, who owns a sailing barge called 'Mandan'. Taking about 30 other trappers with them, they begin to travel 2,000 miles up the Missouri and into the Yellowstone River to seek trade with the Blackfoot Indians, in competition with the Missouri Fur Company. Zeb has brought along Teal Eye, a pretty Blackfoot woman Zeb had found several years earlier after she had escaped from an enemy tribe. She is the daughter of a chief and Zeb plans to return her to her family as a means of establishing trade with the tribe. On the journey, they encounter another Blackfoot that Zeb knows, Poordevil ; they take him along too. Later, Teal Eye falls into the river and is rescued from rapids by Boone. The Missouri Company knows about the threat to their monopoly. One day, it makes its move. A party led by Streak captures Teal Eye and tries to burn the boat, but Frenchy wakes up before the fire causes much damage. Poordevil tracks the enemy and Zeb and Jim rescue the woman. Later the expedition puts in at a company trading post and leaves a warning not to interfere. A week later they repulse an attack by Crow Indians. Jim is separated from the group and shot in the leg. Boone, followed by Teal Eye and Poordevil, finds him, extracts the bullet and waits for his friend to heal. When they rejoin their band, they find Streak trying to buy the boat and the goods on it. Jim compares the bullet dug out of his leg with one of Streak's and finds them to be the same. Streak and his men are killed in the ensuing shootout. The expedition finally reaches the Blackfoot village and begins trading. Teal Eye then tells a very disappointed Jim that she loves him... like a brother. Boone follows her back to her teepee. When he emerges much later, he is surprised to find out he is now married. However, Teal Eye makes him buy her from her father, so that he will be free to leave her any time he wants to. With winter coming on, the men soon begin the long return boat trip and Boone goes with them, abandoning Teal Eye. This cools the earlier friendship between Boone and Jim, who confides to Zeb that unlike Boone he would not have left if Teal Eye had chosen him instead. Later that evening, however, Boone changes his mind and decides to return to Teal Eye, which pleases Jim greatly, and the two men remain friends as they finally go their separate ways.
