The finely braided bullwhip cracks like a shot, sending echoes rattling down the sordid little adobe-walled alley. Luck may have saved his life. If the whorehouse madam hadn't begged him to take it with him, the resentful, drunken troopers might have torn him apart and left him in a dark corner. But hearing the crack of that whip tells them the Lieutenant means business. Grudgingly, a few begin shuffling toward the army transportation they're told awaits them. He swings it overhead again and snaps it hard. It's even louder this time and they move on. Lieutenant Danny Flynn's luck holds long enough to reach the plaza in Piedras Negras. But there it runs out: The promised transportation is nowhere in sight. They've abandoned the lieutenant and dozens of angry soldiers. From a bar near the border crossing, Flynn calls frantically for help, at least an hour away across the Rio Grande. As he waits, IRA agent Maureen Gallamore, disguised in a man's sombrero and serape, appears from the shadows, begging to be saved. She's been posing as a nurse to cover her true purpose: smuggling German saboteurs into the Texas oil fields. Her clients want her for allowing some of them to be ambushed. In loving and consorting with her, Danny has unwittingly provided documents and information useful in avoiding mounted U.S. Army patrols. Suddenly, Flynn sees his predicament in brilliant clarity. Army intelligence at Fort Brown is pursuing an unknown American soldier suspected of funneling materials and information to an agent called the Green Eyed Witch. Saving himself means leaving Maureen to the vengful saboteurs. Helping her means exposing himself as her accomplice. He crosses the plaza to the arriving vehicles.