In 1863, Governor Francis Lubbock (Grandon Rhodes) of Texas has asked Todd Croyden (John Payne) of the Texas Rangers to spring a Union Army spy from a Confederate camp and bring him back. France is trying to seize Mexico and the governor is aiding the latter's defense effort. Guns and ammo are being stolen in Corales, so the governor wants the Ranger and the spy, Whitney Randolph (Dennis O'Keefe), to investigate. Croyden doesn't much care for Randolph, who wins his horse and harmonica gambling on the trail.
While crossing the Rio Grande, they end up held at gunpoint by a woman, Madeline Danzeeger (Rhonda Fleming), whose wagon is stuck. After helping her, they learn that a former German army officer also named Danzeeger, likely her father, is working with a Mexican general, Liguras (Thomas Gomez), for big money. Croyden locates the missing munitions at Basil Danzeeger's (Fred Clark) ranch, but foreman Red Hyatt (Frank Faylen) is suspicious of him. A fire set by Croyden and Randolph destroys the gunpowder, but Madeline turns out to be Basil's wife and takes them prisoner. Red Hyatt tries to kill Croyden by tying him to two wild horses. Randolph dies while saving him.
Madeline has a change of heart, having fallen in love with Croyden, and is shot by her husband. Basil and Liguras end up in a fight to the death fatal to both, and, luckily, it appears Madeline will recover.