Capt. Meriwether Lewis, secretary to President Thomas Jefferson, is visiting the home of Congressman Hancock, whose lovely daughter Julia has won his heart. He is about to reveal his feelings to her when news arrives that the United States has purchased the Louisiana Territory from France, thereby more than doubling the young nation's size. Lewis greets his old friend, Lt. William Clark, who has just arrived at the Hancock home, but then learns that the President has recalled him to Washington. At the White House, Jefferson places Lewis in charge of a military expedition that is to explore and chart the new territory from the Mississippi to the Continental Divide. Jefferson orders Lewis to continue even beyond the boundary of the purchase, however, proceeding, if possible, to the Pacific Ocean. Lewis wants Clark to share the command of the expedition, but when he returns to the Hancock estate, he learns that his friend and Julia have become engaged. Because Clark was unaware of Lewis' feelings for Julia, the captain forgives him, but his pain at losing Julia is evident. Later, in Wood River, near St. Louis, the two meet the flinty Sgt. Cass, along with most of the men who will accompany them, and Clark is annoyed to learn that a paperwork error has delayed his promotion to the rank of captain. The expedition travels up the Missouri in a large keelboat, mapping the river as they go. Upon arriving at a Minitari Indian village, Lewis assures the chief that the United States hopes for peaceful and friendly relations with the tribe. Although the tribal leader publicly acknowledges the sovereignty of the United States, he secretly plans for his warriors to ambush the expedition when it resumes its course. Assisting him is a French trader named Charboneau, who agrees to lead Lewis and Clark into the trap in exchange for a captured Shoshoni slave named Sacajawea. Seeing a chance to escape slavery and return to her people, Sacajawea asks if she might serve as the expedition's guide, but Clark distrusts Indians and refuses her request. After the white men leave, Sacajawea witnesses a war dance and realizes that the white men are heading into an ambush. She steals a horse and secretly rides ahead of the departing war party, arriving at the expedition's camp in time to prepare the soldiers for battle. After they defeat the Indians, Clark invites her to remain with the expedition. Continuing up the Missouri, the party splits up, with Lewis' group exploring one fork of the river, and Clark's the fork recommended by Sacajawea. After Sacajawea leaps into the strong current to rescue Clark's book of maps, he begins to call her Janie, a name, he remarks, that means "beautiful." Later, Clark falls ill, and as Sacajawea nurses him back to health, she realizes she is in love with him. Following his recovery, Clark tries to bolster the flagging spirits of the men, but when he dances with Sacajawea, Charboneau attacks him with a knife and claims that she is his. Sacajawea later declares that because he fought for her, Clark now possesses her. Clark gently rebuffs her because of her race, and even though he is unable to explain his reservations, she promises to wait for his marriage proposal. With both parties now reunited, the expedition finally reaches the village led by Sacajawea's brother Cameahwait. The Shoshonis are grateful to the explorers for returning their abducted sister and agree to provide horses for their journey over the mountains. Opposed to this plan is Wild Eagle, the warrior to whom Sacajawea had been promised before her capture. That night, Sacajawea again offers herself to Clark, but he allows her to remain in the tent only to prevent her from being given to Wild Eagle. Cameahwait guides the expedition over the mountains to the river "that leads to the great salt water." A messenger is sent ahead of the party to advise the Nez Perce that the white men are friends. Wild Eagle, however, kills the messenger and paddles into Nez Perce country intending to lay a trap. Meanwhile, Lewis accuses Clark of disregarding the feelings of both Sacajawea and Julia, and demands that the Shoshoni princess be sent home. Seeing that Clark intends to bring Sacajawea along, Lewis uses his superior rank to assume full command of the expedition, and orders Clark to leave her onshore. As the party canoes downriver, Sacajawea keeps pace with them by running along the shore until she collapses from exhaustion. Clark kisses her and places her in his canoe, whereupon Lewis threatens to have him court-martialed, and the two brawl. Down river, the party encounters the Nez Perce ambush devised by Wild Eagle. During the subsequent battle, Clark kills Wild Eagle, and Lewis removes the rope blocking the river. When the expedition finally reaches the Pacific Ocean, the slain are buried, and Lewis claims all of the land between the Rockies and the Pacific for the United States. Back in Washington, D.C. in 1806, Lewis and Clark introduce Sacajawea to President Jefferson. Later, Julia and "Janie" discuss the responsibilities of being a white man's wife, and Julia realizes that Clark loves the Shoshoni woman. When Julia learns that Lewis intends to have Clark court-martialed on her account, she asks Lewis to forego his plan, and to Clark's surprise and gratitude, Lewis desists. At a White House reception later that day, Julia tells Clark that "Janie" has returned to her own people, reading him a letter that Sacajawea dictated. In it, she explains that although the white people were kind to her, they were not her people, the United States not her country. "Have happy memories, like ours were, my love," she writes, "all the days of your life."