A ruthless Oriental villain, Dr. Fu Manchu (played by Boris Karloff), seeks to acquire the ceremonial mask and sword of Genghis Khan. With these sacred relics, Dr. Fu Manchu plans to lead the "teeming hordes of Asia" in wiping out the white race and ruling the world. A group of scientists, hoping to obtain the relics for a British museum, embark on an expedition to excavate the tomb of Genghis Khan in the Gobi Desert. Scotland Yard's Nayland Smith (played by Lewis Stone) joins the group to prevent the relics from falling into Dr. Fu Manchu's hands.
Dr. Fu Manchu resorts to kidnapping, murder, and torture to obtain the mask and sword of Genghis Khan. Nayland Smith and Professor Von Berg (played by Jean Hersholt) are sentenced to death in a crocodile pit and by impalement, while young Terry Granville (played by Charles Starrett) is destined to become a slave to Dr. Fu Manchu's twisted daughter, Fah Lo See (played by Myrna Loy). However, Nayland Smith and Von Berg escape and use Dr. Fu Manchu's own death ray to destroy him. The relics are sent to the bottom of the sea to prevent them from falling into the hands of future tyrants.