A strange epidemic is sweeping the Riviera. In desperation the French authorities call upon Dr Petrie to find an answer. During this crisis, a mysterious siren on the beach captivates Alan Sterling. She tells him her name only—Fleurette—and flees. When Petrie's lab cultures show up sleeping sickness and plague, they call in Sir Denis Nayland Smith. It is not long before their investigations lead them to Fleurette—and to Dr Fu Manchu...