In London, Ronald Petrie and his fiancée, Joan Marshall, meet celebrated Chinese dancer Princess Ling Moy and discover that her manager Morloff lives next to them. At the same time, Chinese detective Ah Kee has traced Fu Manchu to London, even though Fu Manchu was reported dead twenty years earlier. On his advice, Scotland Yard warns the Petrie family, against whom Fu Manchu had sworn vengeance. Fu Manchu infiltrates the Petrie house, however, and kills Ronald's father Sir John. Mortally wounded by Ah Kee, Fu Manchu escapes through a secret tunnel into the home of Morloff, who is his comrade. Before he dies, Fu Manchu reveals to Ling Moy that she is his daughter and commissions her to complete his revenge against the Petries, whom he wrongly blames for the death of his wife and son. Although Ling Moy has fallen in love with Ronald, she attempts to kill him, with the help of Morloff's servant, Lu Chung, but they are thwarted by Ah Kee and the butler, Rogers, who are unable to identify the figures fleeing in the dark house. Unaware of Ling Moy's relation to Fu Manchu, Ah Kee has fallen in love with her, and he beseeches her to return to China with him. Ah Kee, however, is held prisoner by Ling Moy and Morloff after he discovers his wine is drugged, and Ling Moy reveals her heritage. Ronald and Joan are then held captive in Morloff's basement, and Ling Moy prepares to torture Joan with acid unless Ronald kills her himself. Bound in a turret, Ah Kee sees Sir Basil Courtney of Scotland Yard on the grounds and leaps out of a window to warn him of Ronald's danger. Ling Moy escapes into the Petrie house, and Rogers and Ronald rush there, finally aware of the passageways. Rogers shoots Morloff's dangerous chinese servant, Lu Chung, when he emerges, and Ronald grabs Ling Moy. She is about to kill him, however, when Ah Kee, although severely wounded from his fall, shoots her, then dies next to the woman he loved.