In 1810 in New York, John Jacob Astor plans a two-year fur expedition into the Oregon wilderness. He tells Scottish fur traders Duncan MacDougall and Angus McKay that he is sending with them his top man, Robert Stevens, a seemingly mousy accountant. The crew of the expedition is to be manned by a group of rough-and-tumble French-Canadian voyagers led by Ovide de Montigny. At a local café, Ovide meets naïve singer Julie Morgan, whom he seduces by promising to take her to Paris. When she asks to go with him when he sails the next day, he lies simply tells her that the ship is too small, without revealing its true destination. The ship Tonquin sets sail, under the stern discipline of Captain Jonathan Thorn. Once at sea, Robert discovers that Julie has stowed away on the ship, still thinking it is going to France. Thorn discovers Julie and, refusing to listen to the truth, announces to the crew that she was smuggled aboard by Robert and that she is to be treated like a cabin boy. Ovide fails to admit his involvement in the matter, and bitter rifts develop between Robert and Ovide, and between Robert and Thorn. Later in the voyage, Julie finally confronts Ovide, and he uses his manly charms to convince her to meet him on deck that night. When Thorn sends Robert to summon Julie, she falls overboard, but is saved by Ovide. Thorn soon becomes infatuated with Julie, and changes course to the Falkland Islands to take on water. At first, Thorn refuses to allow Julie ashore, but Robert, MacDougall and McKay convince the captain to let her go with them. Ovide and Julie sneak off together, and she asks him to marry her. When he refuses, she realizes the truth about the French-Canadian's intentions and runs off. Thorn decides to sail without them, and when he refuses to listen to Robert's pleas to wait, the young accountant threatens to kill the captain. After Ovide and Julie are safely aboard the ship, Robert is arrested for mutiny. Ovide then confesses all to Thorn and states that he will finally agree to marry the singer. When Thorn questions his motives, Ovide suggests that the captain may be more interested in her than he wishes to acknowledge. Upon arriving at the mouth of the Columbia River in Oregon, Thorn agrees to release Robert, but warns him not to return to the ship unless he wants to face trial for mutiny. After two months, the fort is almost completed, but the fur trading is going very slowly. Thorn tells Julie that she can live ashore, as Robert, Ovide and McKay are leading an expedition into the interior. Before they leave, however, Robert and Ovide get into a fight over Julie, during which Ovide falls, breaking his leg. After three months without word from Robert's river party, Thorn decides to take command of the expedition and moves the ship two hundred miles north to deal with another tribe of Indians. In the meantime, Robert returns, only to learn that Thorn is heading into a trap devised by the deceitful Indian Lamazie. Realizing that Julie is aboard the ship, Robert and Ovide travel overland to try to warn the ship. Robert is injured trying to traverse a waterfall, but continues with Ovide, despite his impairment. Their canoe arrives just as Lamazie's tribe is about to spring their trap. Robert sneaks Julie off the ship, then warns Thorn just as the Indians attack. In the ensuing mêlée, Ovide and Thorn are mortally wounded. Realizing that defeat and death are imminent, Thorn blows up the ship once Robert and Julie are safely ashore.