The Ace of Scotland Yard

From the Universal synopsis: Retired Inspector Blake of Scotland Yard learns that Lord Blanton has acquired the sacred "Love Ring"---a cursed jewel that has brought grief to its previous owners. Prince Darius seeks to obtain the ring, and the mummy of a princess of an ancient country to return them to his homeland. Blake and his servant, Jarvis, find a card---The Queen of Diamonds---which is the mark of the most dangerous woman in the underworld. Mary Duveen , the Blanton housekeeper, is really the Queen, disguising herself as a spectacularly ugly crone. Darius comes to believe that Lady Diana Blanton, Lord Blanton's daughter, is the reincarnation of Princess Xah Fari, and sets his sights on her also. His hypnotic influence soon has the girl experiencing dreams of a vanished civilization. Mary acquires the ring and Jarvis recovers it and Mary steals it back and plants it on Lady Di, and then lures her to a Limehouse den but Blake and Jarvis follow and rescue her and the ring. Learning that Diana intends to give the ring to Darius, to end the effects of the curse, Blake substitutes a copy. He accuses Mary of theft, and reveals evidence that the real Queen of Diamonds is in prison, and she admits to being an agent for a wealthy American collector who seeks the ring. She assumed the guise of the Queen in order to gain the cooperation of the underworld. Blake persuades her to assist him against Darius, who he believes to be "Diamond" Donovan, an international criminal. But Darius convinces Diana that Blake and Mary are working against her, and she turns against the Scotland Yard man.