In 1803, the United States Congress approves the Louisiana Purchase from France, opening a bold push toward the Pacific. Meriwether Lewis is summoned to the White House, where Thomas Jefferson asks him to lead an expedition to cross the newly claimed lands and reach the ocean beyond. Lewis agrees, and he asks his close ally to join him at an equal rank, inviting William Clark to come along as a co-leader. This partnership is strained when Lewis returns to Virginia to tell his beloved Julia Hancock that Clark has proposed to her, creating early tension between the two men.
At Wood River, Lewis and Clark meet with Sgt. Gass and the volunteers before they depart in a keelboat. A crucial snag arises: Clark discovers he has not been officially promoted to captain as promised, while Lewis insists the rank is rightly his. The journey begins, moving north along the Missouri River, and the expedition soon encounters the Minnataree tribe. A peace offering is exchanged, and a troubling new development arrives in the form of a kidnapped Shoshone woman named Sacagawea, who desires to accompany the expedition as a guide and to return to her people. Charbonneau, a French agent associated with the Minnataree, joins the group, but behind the scenes he plots with the Minnataree to betray the expedition.
Sacagawea proves her worth when she learns of the Minnataree plan to strike and escapes on horseback to warn the captains. Impressed by her courage, the leaders invite her to stay and help. After a division at a fork in the river, Sacagawea accompanies Clark’s team and even saves Clark’s journal from tumbling into the water. Her presence helps Clark overcome a fever, and a deepening bond forms between them as he reinterprets her name, calling her Janey.
The expedition progresses and Sacagawea is soon reunited with her people, meeting her brother Cameahwait along the way. Lewis grows increasingly uneasy, convinced that Clark’s growing affection for Sacagawea imperils the mission. He presses to have Sacagawea leave, though Clark resists this demand. Sacagawea’s presence is reaffirmed when she chases the expedition downstream and re-joins the party, prompting Lewis to contemplate harsher action upon their return home.
Tensions among the crew intensify as the journey takes them far beyond the Louisiana territory. After the deaths of several crew members, Sgt. Gass informs the captains that morale is fraying and that the group should consider turning back, but Lewis remains focused on reaching the Pacific. The decision is made to press on, and the explorers finally reach the ocean. Clark announces that Sacagawea will accompany him to Washington, while Lewis reveals in his journal that Clark’s insubordination has not gone unnoticed.
In 1807, Lewis and Clark return to the White House and are greeted with accolades from Thomas Jefferson. Clark introduces Sacagawea to the president, who thanks her publicly for her contributions to the journey. The two women share a moment as they learn how each feels about Clark. Faced with the political and personal complexities of their voyage, Lewis chooses not to punish Clark for the past misgivings. He even removes the final pages of his journal before submitting it to Jefferson. The expedition closes with a gala, where Julia Hancock informs Clark that Sacagawea has left a goodbye letter for him, signaling her return to her people and the end of an era.
