Stephen Stephani leaves Nordhoff with his daughter Mary to visit Zandria, an enemy country, where he tries to steal the war plans of the hostile nation. There, Mary meets Paul Ekald, a Zandrian captain, falling in love with him at first sight. While Mary remains in Zandria for the moment, Stephani returns to Nordhoff. Meanwhile, Vesta, Mary's illegitimate half-sister, has managed to get hold of important war plans stolen from Count Wenzel. But, to get them, she had to kill the count. While trying to reach Nordhoff, she is stopped at the border. There, she also finds Mary waiting to cross the border. Vesta convinces Mary to exchange her passport with her, in order to allow her to leave the country. She thus brings to Nordhoff the plans drawn with invisible ink on her petticoat. Meanwhile, Mary is arrested and, accused in place of Vesta, sentenced to be shot. Vesta, after having fulfilled her mission and learning of her sister's fate, goes back and redoes the exchange with her, allowing her to save herself. After some time, peace is declared and Mary can reunite with the man she loves.