During the Civil War, Union forces stationed in Colorado are hampered by raiders who have been stealing their horses as they travel through the mountains. When several commanding officers begin to suspect a traitor is revealing troop movements, Maj. Lex Kearny (Gary Cooper) goes undercover to investigate. However, the assignment strains Kearny's relationship with his wife, Erin (Phyllis Thaxter), after he chooses to continue the operation instead of searching for their runaway son.