In the midst of the Civil War, Union Captain Roper is in command of Fort Bravo, a remote desert prison camp in the southwestern United States that houses Confederate captives. Known for his strict discipline and unyielding nature, Roper has a reputation for preventing escapes through the treacherous, arid surrounding landscape that is said to be as deadly as the prison itself, with inhospitable terrain and roaming Mescalero Indians. The prisoners, resigned to a life behind bars, entertain themselves with plots of escape they rarely attempt due to the perilous conditions.

The monotonous routine of the fort is disrupted by the arrival of Carla Forester, a Southern belle with ulterior motives. Charismatic and cunning, she quickly captivates the men of Fort Bravo, including Captain Roper. While ostensibly visiting Fort Bravo to see the fort's commanding officer, she is covertly aiding a group of Confederates led by Captain Marsh in planning a daring escape. As the prisoners implement their harrowing escape under the cover of darkness, loyalties are tested, and Roper's ironclad facade is challenged by the unfolding events and his growing affections for Carla.