Die unsichtbare Front.

During World War I, Ellen Lange runs away from boarding school to her brother Rolf in Kiel. Rolf, a vice-helmsman, tries to send her back, but she goes to Berlin and finds a job at a music store through “Aunt Jenny.” Her boss sends her to Copenhagen with a false passport to deliver a violin containing stolen papers. In Copenhagen, she meets Erik Larsen, a German secret service agent, and joins the service to atone for her mistake.