A strange epidemic sweeps the French Riviera - a biological weapon created by Dr. Fu-Manchu. Dr. Petrie is called upon by the French authorities, and when the truth emerges, Denis Nayland-Smith is summoned to help stop his arch-foe before he can succeed in spreading his plague across Europe. As they struggle to contain the horror, Petrie's friend, the botanist Alan Sterling cannot stop thinking of the mysterious Fleurette, unaware that the beautiful girl he chanced upon was raised by the emperor of evil himself, Dr. Fu-Manchu.

One of the finest novels in the series, its biological theme is a precursor to Ian Fleming's On Her Majesty's Secret Service, and Fu-Manchu himself is the prototypical Dr. No.