Based on a story by Sax Rohmer, the British struggle to keep the evil Dr. Fu Manchu from getting the sword and mask of Genghis Khan.
British agent Nayland Smith (Lewis Stone) sends archaeologist Lionel Barton (Lawrence Grant) to find the tomb of Genghis Khan before Fu Manchu does. Barton plans to take professor Von Berg (Jean Hersholt) and McLeod (David Torrance), but he is abducted in the museum. Nayland tells Barton's daughter Sheila (Karen Morley) that her father was captured by Fu Manchu. Dr. Fu Manchu (Boris Karloff) offers Barton a million pounds, women, and even his daughter Fah Lo See (Myrna Loy) for information. He tortures him under a huge bell. The expedition of Von Berg and McLeod digs and finds the tomb. Von Berg reads the curse and opens the tomb, and McLeod removes the mask.
In an Asian court Fu Manchu introduces Fah Lo See, who describes her vision of the East conquering the world. Nayland is eager to get out of the country and sets McLeod to watch at night. An Asian gets to the window and throws a knife, killing McLeod. They bury him. Nayland has Terry (Charles Starrett) watch. A messenger tells Terry that Fu Manchu has Barton and wants the sword and mask. Sheila persuades Terry to take them to Fu Manchu, who tests the sword with an electric current that burns it up. Fah Lo See orders Terry whipped, and the corpse of Barton is delivered to Sheila and Nayland. Nayland, who had replaced the sword, goes to an opium den, throws a lamp in a Chinese saloon, and falls into a cellar. There Fu Manchu with a gun takes him to Terry and Sheila. Fu Manchu aims to wipe out the white race and has Nayland taken away. He gives Terry a serum from a spider and a snake to control his mind.
Terry returns to Sheila and Von Berg, saying that Nayland told them to bring the mask and sword. Sheila says Terry is drugged; but Von Berg gives them to Terry in the rain. Terry runs off with them as Von Berg and Sheila are captured. Fu Manchu tests the sword with the current, plans to sacrifice Sheila and to kill Von Berg and Nayland. Sheila tries to waken Terry, and he embraces her. Fu Manchu has them taken away and tells Sheila that Terry will be drugged to become Fah Lo See's slave. To his people Fu Manchu speaks with the mask about killing the white race and taking their women. Nayland escapes from crocodiles, and Terry fights the guards. They release Von Berg. Nayland and Von Berg direct the current at Fu Manchu, while Terry takes the sword and rescues Sheila. In the final scene on a ship Nayland throws the sword in the ocean.
This horror story reflects fears of Asian imperialist aggression at a time when Japan had just taken over Manchuria with military force and was likely to continue expanding its militaristic empire.