Sir John Manders, an English government agent is on a secret mission in France disguised as a Dutch diamond merchant. Whilst at a party given by Madame Sans-Gene and her amusing friend, Cabouchon, Sir John is recognised by French woman Louise Varennes, whom he had known in London when she was living there as a refugee during the Revolution. She does not give him away. However, her lover, Captain Edouard Labouchere, slightly tipsy gives away information regarding a plan of Napoleon’s against England (thus proving the much-advertised wartime slogan “careless talk costs lives”) and he is arrested. Sir John, who also loves Louise, plans for her sake Edouard’s escape from prison. After a great deal of plotting, court intrigue and sword-play, he finally manages to get the young couple safely away from France, making Louise promise to deliver his message to the Prime Minister in London, revealing Napoleon’s plan, an act which will ultimately lead to Nelson’s victory at Trafalgar.