While Paul Sterling is ordered to work in the botanical research room of the underground laboratories, Nyland Smith is sent back to the outer world to carry on his work which the insidious doctor plans to direct towards the interests of the Sci-Fan. However, Fu Manchu is ignorant of the fact that his daughter Fa Lo Swee has rendered the injections harmless and warned Smith and Sterling that they must pretend. Smith returning to Monte Carlo immediately contacts Weymouth. He learns that Dr. Petrie who has died of a mysterious plague known as the purple shadow has already been buried. Smith orders the grave reopened and when the coffin is opened Smith and Weymouth discover no trace of Petrie's body. Smith decides to return to Fu Manchu's laboratories where Paul Sterling awaits his summons for duty...