The Swedish Sphinx portrays Tania, a pre-World War I Russian agent who uses seduction as a weapon to steal military plans from a handsome Austrian officer (Conrad Nagel). But duty cannot conquer Tania's heart. She realizes she loves the now-disgraced Austrian and launches a plan to retrieve and return the documents. Danger. Treachery. Romance. Elegance. Courage. They're the tools of the trade of The Mysterious Lady.