A Union cavalry brigade, led by Colonel John Marlowe (John Wayne), is sent on a raid behind Confederate lines to destroy a railroad and supply depot at Newton Station. Ironically, before the war, Marlowe had been a railroad building engineer. With the troop is a new regimental surgeon, Major Henry Kendall (William Holden) who seems to be constantly at odds with his commander. Kendall is torn between the duty and the horror of war. Complicating matters, while the unit rests at Greenbriar Plantation, Miss Hannah Hunter (Constance Towers), the plantation's mistress, and her slave Lukey (Althea Gibson) eavesdrop on a staff meeting wherein Marlowe discusses his plans. To protect the mission, Marlowe is forced to take the two women with him. Initially hostile to her Yankee captor, Miss Hunter gradually warms to him. In addition to Miss Hunter, Marlowe also has to continually contend with Col. Phil Secord who doubts Marlowe's orders and command decisions. Several battles later, including a fire-fight which results in the death of Lukey, and a skirmish with Boy Cadets from a local military school (based on the real-life Battle of New Market), and with Confederate forces in pursuit, Marlowe and his command reach a bridge which must be stormed in order to access the Union lines. Dr. Kendall is forced to choose between remaining behind with some badly wounded men (and being captured with them), or leaving the men without medical care until the Confederates arrive. Marlowe and the remainder of his troop escape back to Northern lines.