The Mystery of Mr. Wong is a 1939 mystery film directed by William Nigh. The 'Eye of the Daughter of the Moon', the largest star sapphire in the world, has been stolen from a museum in China. A wealthy gem collector, Brandon Edwards (Morgan Wallace), has obtained the sapphire. Edwards invites Mr. Wong (Boris Karloff) to his home for a dinner party and confides in him that he has the sapphire and feels his life in danger because of it. When Edwards is murdered during a party game, the sapphire disappears. Mr. Wong sets out to find it, but a string of murders leave him unsure of where to find the diamond. But as people begin dropping like flies, he soon narrows down the list of suspects. Can Mr Wong find the rare gem and return it to China, or will it be forever lost to the thieves and murderers that covet it?