Marthe and Stephan are two Belgians working in a German hospital, during the First World War. This is their cover: they are in fact spies for the Allies. After blowing up an ammunition dump, Marthe puts herself in more danger by accompanying the German Commandant to Brussels, where she hopes to gather vital information about the Kaiser. Her mission becomes fraught with danger and it gets harder and harder for her to hide her true identity. Knowing this, Stephan sets off to help her, but will he be too late? Based on the memoirs of Marthe Cnockaert, a real Belgian spy for the UK and her allies, during the First World War.