A little knowledge is a dangerous thing, but obtaining that knowledge is even more dangerous for a Civil War spy. Marion Davies portrays the Union agent codenamed Operator 13, whose work behind enemy lines grows more precarious when she violates the first rule of espionage: never fall in love with the enemy. The fact that Gary Cooper ' tall, handsome and striding into career greatness ' plays the Confederate officer she falls for certainly helps explain the operative's vulnerability. He and Davies (who three years later left movies for the splendor of the San Simeon estate she shared with William Randolph Hearst) both earned critical plaudits, as did the film's battlefield sequences between the clashing Blue and Gray.