A beautiful British intelligence agent attempts to reveal the identity and motives of a powerful German spy during World War 1.

During World War I, English woman Frances Hawtree, a German spy known as "ZI," is sent to the estate of Sir Winston Chamberlain, first Lord of the Admiralty, to uncover information that will alert the German navy about the Atlantic crossing of the First American Division. Posing as a nurse, Frances pretends that she is returning the belongings of the Chamberlain's eldest son, who she says died at the same German hospital she escaped from. Also at the estate is Chamberlain's other son, Arthur, who comes home on leave and falls in love with Frances. Unknown to the family, their seemingly humble butler, Valdar, also is a German spy and the person who masterminded Frances' mission. Valdar is charmed by Frances, but suspicious of her, and eventually suspects that she is a double agent, whose true loyalties are to the British government. When Valdar is sending a radiogram that to the German U-boats on the landing point of the American fleet, Francis warns Valdar to stop and then shoots him when he ignores her and continues sending the message.