Sir Gregory Hale returns to London from Mongolia with a mysterious Tulun-Nur chest that holds the "key to India," a vital secret of Dr. Fu-Manchu's notorious Si-Fan organization. Unfortunately, Hale is murdered before he is able to disclose the secret to Nayland Smith. The Burmese police commissioner and Dr. Petrie launch a mission to affront the brilliant but deadly master criminal before he succeeds in his malignant and fantastic plot to take over the world.

In pursuit of the "Devil Doctor," Smith and Petrie must escape numerous assassination attempts and battle an insect army. They are faced with multiple mysteries, including the Zagazig Code, the Shrine of Seven Lamps, and the Chapel of Satan. The future of the free world depends on the success of Smith and Petrie defeating the villainous Hand of Fu-Manchu.