Academy Award winner Gary Cooper stars as a Union officer in the United States' Civil War who goes undercover-pretending to be court-martialed-to uncover who is stealing army horses and selling them to the South before they can hijack a delivery of the new Springfield Rifle. While delivering a shipment of horses desperately needed to mount a big offensive in the United States' Civil War, Union Major Lex Kearny (Cooper) is overwhelmed by a band of Confederate raiders. Rather than risk the lives of his men, he sounds retreat, leaving the precious cargo behind-for which he faces a court-martial. But this is only a ploy to send Kearny underground to find who has been stealing horses from the Union Army. Kearny's mission remains so secret that even his wife (Phyllis Thaxter) believes he has gone renegade, and she surrenders evidence that implicates him as a Southern spy. Now, can Kearny stop the gang of traitorous horse thieves before he is sentenced to death for espionage?