1917. American boys have joined the Great War in Europe. On the homefront, mysterious occurrences like ships catching fire and factories blowing up have people wondering: is this German sabotage?

Hilda Lessing's family has been split apart due to the death of her father, and her uncle has found her a job doing housework on a friend's farm. Mr. and Mrs. Schwartz and their workers are German and a young train engineer whom she befriends on the way to the farm jokingly warns her to "Watch out for spies!"

Noting her concern, he suggests a signal. In case of trouble, she will hang a red scarf out of her window that he can see as he drives his train past. Hilda's life on the Schwartz farm is a misery. She toils from dawn to dusk and her employers and the farmhands are crude. Exhausted and hopeless, she comes to realize that the menace that seems to surround her involves their preparations to blow up a nearby railroad bridge.

Now on the alert, the clever girl makes plans to thwart them. Hilda may not be a soldier but she can still fight for her country! She has one huge advantage: none of them knows that she is completely fluent in German and can understand every word they say....