Three Faces East (1930)

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 was killed in combat. 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 realizes that she is a double agent, whose true loyalties are to the British government. When Valdar finds her sending a radiogram that will thwart the Germans' plan to sink the American ship, she momentarily hesitates, then kills him.