A Chinese warlord threatens explorers in search of the key to global power.

Nayland Smith begs his friend, Sir Lionel Barton, to find the tomb of Genghis Kahn before it is discovered by Chinese scientist Dr. Fu Manchu, who will use its contents for his own evil purposes. Just after Sir Lionel explains the expedition to his colleagues at the British Museum, however, he is kidnapped by Manchu's emissaries. When Sir Lionel's daughter Sheila hears of her father's capture, she tells Smith that she plans to search for the tomb herself. Meanwhile, at Manchu's headquarters in the Far East, the doctor tries to cajole and bribe Sir Lionel to reveal the secret, and even offers him his daughter, Fah Lo See, but Sir Lionel refuses. While Sir Lionel then undergoes Manchu's fiendish "torture of the bell," Sheila and the men of the expedition find the tomb and unlock its doors, despite a warning of a terrible curse. When the group meets up with Smith, he warns them that Manchu may strike at any moment, hoping to gain possession of the sword and mask of Genghis Kahn, which they found in the tomb. That night, expedition member McLeod is killed by a dagger thrown through a window. The next day, Terrence Granville, Sheila's fiancé, finds a human hand wearing Sir Lionel's ring, after which one of Manchu's underlings invites him to meet Manchu. Though Terry knows it is foolish, he agrees to go to Manchu's headquarters, carrying the sword and mask. Fah Lo See, who is attracted to Terry, orders her father's men to whip him when the sword turns out not to be geniune. She wants to make love to him later, but is stopped by Manchu, who has other things in mind for Terry. Manchu then has Sir Lionel's body delivered to the expedition's compound and Smith sadly reveals that he had made the phony relics to fool Manchu. Smith tells his colleague, Von Berg, that he must go away, then enters an opium den, where he sees a man with the Tattoo of Manchu on his shoulder. Smith follows the man and finds the secret entrance to Manchu's headquarters. After Manchu discovers Smith, Smith demands the release of Terry just as Terry is about to be injected by Manchu with a serum that will make him totally subject to the doctor's will. Manchu prepares the serum, derived from various reptiles and deadly insects, and tells Terry that it is the smallest dose, so that he will be himself again for Fah Lo See. Soon Terry, under the influence of the drug, goes to Sheila. Sheila suspects that Terry has been drugged when he blankly asks for the real sword and mask, but she and Von Berg still go with him and are captured by Manchu's men. At Manchu's headquarters, Sheila sees Fah Lo See with Terry and tries to get him out of his stupor. He does awaken, but Manchu orders Sheila taken away and prepared as a human sacrifice to the gods. The next morning, as Sheila lies on the sacrificial table, Smith breaks free from his crocodile infested cell and frees Terry. Together they then free Von Berg and tamper with Manchu's electricity machine, sending an electrical charge to the sword, apparently killing Manchu. While Terry rescues Sheila, Smith and Von Berg use the machine to send shocks to Manchu's men. On the boat back to England, Smith decides to throw the evil sword overboard.