In 1846, the United States, under President James K. Polk, plans to extend the country's borders to the edge of the Oregon territory, despite conflicting claims by the British. Facing possible war, Polk orders soldiers disguised as civilians into the territory. Newspaper publisher James G. Bennett hears rumors of this and dispatches reporter Neal Harris to investigate. Harris joins a wagon train headed by Seton. Also in the train are Capt. George Wayne and soldiers Brizzard and Ellis, all in civilian disguise; beautiful Prudence Cooper, her father Richard, grandmother Maria and brother Jeremiah; and Zachariah Garrison, who is transporting apple trees to Oregon. Along the trail, the wagon train discovers the remains of other settlers, who were attacked by Indians. Seton reveals that he had been married to a Sioux woman who was killed by Arapahos, and the same Indians cut out the tongue of Seton's assistant, Jesse. As the journey continues, Harris begins to suspect that Wayne, Ellis and sadistic drunk Brizzard are soldiers. The settlers' water supplies run low, and just as it starts to rain, ailing Maria dies. Prudence and Wayne fall in love. When Harris interrogates Wayne, the soldier evades his questions, stating that it is dangerous to ask them. Later, they find an Arapaho warning signal and shortly after, Seton is killed. Jesse shoots the Indian who killed Seton, and Wayne takes over as leader of the wagon train. When Harris announces that he is going to notify his paper that soldiers are being sent to the Oregon territory, Wayne assigns Brizzard to guard him. That night, however, Brizzard gets drunk, and Harris steals his horse and leaves the train. Wayne then sends Brizzard to follow him. Harris reaches Fort Laramie and learns that the soldiers have abandoned it. Trapper Gabe Hastings then arrives with his half-Indian daughter Shona, and Harris defends the woman when her father brutally attacks her. Harris sees the wagons approaching and offers to pay Hastings to send his dispatch to the newspaper. Hastings takes his money and then offers to hide Harris in Shona's village. At the fort, Wayne learns that the Oregon boundary dispute has been settled and that the United States is now at war with Mexico. Before he leaves for Mexico, Prudence promises to wait for him. Meanwhile, Harris, Shona and Hastings arrive in the Indian village and see Brizzard, tied up and left in the sun. On Hastings' orders, Harris is also taken prisoner. Hastings denounces the settlers for driving the Indians off their land and threatening the livelihood of trappers like him. That night, the Indians hold a war dance, and in the morning, they take Brizzard away. After they leave, Shona, who has fallen in love with Harris, kills his guard and cuts him free. Together they warn the fort that Indians will be attacking. The soldiers, who have not yet left, supply the settlers with guns. As Indians surround the fort, Brizzard appears in a wagon, and Wayne orders the gates open, not knowing that the wagon is filled with Indians. After an intense battle, during which many settlers, including Cooper and Garrison, are killed, the Indians are driven off. Shona renounces her people and continues to Oregon with Harris, who has quit his job on the newspaper. In their wagon is the last of Garrison's apple trees.