The ship Lusitana has been torpedoed and a stranger has given a young girl a packet contaning an important document. The stranger dies, and the girl, whose name is Jane Finn, disappears, and the packet is forgotten. Until, that is, a labor unrest starts in England and if the papers fall into the wrong hands it could prove deadly to the government. Tommy Beresford and Prudence Cowley (nicknamed Tuppence) know nothing of this when they unexpectantly meet.