Forced by Nyland Smith's activities Dr. Fu Manchu is about to about to abandon his laboratories at St. Clare de la Roche in the south of France. When Smith trusting that the insidious doctor still believes him to be a member of the Sci-fan walks in to a trap. Fu informs Smith that his and Dr. Petrie's lives are no longer of interest to him but that he intends to transform Paul Sterling, Petrie's assistant, in to a zombie since Sterling's botanical knowledge is valued by the oriental. Smith and Petrie kept prisoners in one of the underground rooms try to escape but find the corridors infested with death dealing germs and insects. Donning chemical masks they traverse the deserted laboratories and make their escape. Captain Duboire informs Smith that no one has left the place and Smith accompanied by Petrie and the officers reenters. He discovers a secret door which reveals steps leading down to a subterranean lagoon where they find themselves trapped in...