Colonel John Marlowe (John Wayne) leads a Union cavalry on a raid behind Confederate lines to destroy a railroad and supply depot at Newton Station. Major Henry Kendall is the regimen’s doctor, who doesn’t believe in war, and does not get along with John.
At a stop at Greenbriar Plantation, the cavalry arrests Miss Hannah Hunter and her slave, Lukey, after they eavesdrop on the commanders. Since they learn of the Union’s plan to head to Baton Rouge, Marlowe has no choice but to take the women with him, or kill them. While Miss Hunter hates her Union captors, she eventually falls for the Colonel.
After a number of battles, skirmishes, and raids, many men are lost on both sides. The Rebels manage to shoot Lukey dead during a gorilla attack on the troops. Along the way, the U.S. Cavalry manages to destroy the supply line, and they then head south for Baton Rouge with the Confederates hot on their tail.
In the end, the only thing that stands between the Union cavalry and success is one lone bridge. So Marlowe orders his men to rig it with explosives. Major Kendall refuses to leave the side of the badly wounded Union soldiers, even though it will result in his capture.
Upon Marlowe’s men setting the proper explosives, Marlowe says his goodbyes to Hannah and Kendall, and then blows up the bridge. Hannah and Major Kendall are now trapped on the side of the river as the approaching Confederates, but Marlowe and his men ride victoriously off to Baton Rouge.