Unneutral Murder by Hulbert Footner

The liner on which Lee Mappin was travelling on what was to be one of his most dangerous and exciting missions slid unescorted from New York, her sides brilliantly flood-lit to display the neutral flag of Portugal. Among the passengers who crowded every cabin were several in whom Lee took more than a mild interest; the good-looking, smiling young Ronald Franklin, for instance, who said he was an American agent for Swiss watches; John Stanley, going out to the hard-pressed U.S. Legation in Lisbon, and his young wife Vera, daughter of a wealthy tobacco nabob; Kate McDonald, the smartly dressed but slightly pseudo-Scottish woman. Whatever the divers purposes which prompted their voyage to Europe their immediate activities were going to be very much the concern of Lee Mappin.