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Produced by the British Apex company, the 4-reel Mystery of the Diamond Belt wasted precious little time on inessentials. The film represented one of the first screen appearances of pulp-novel sleuth Sexton Blake, here depicted as a Sherlock Holmes in a bowler hat. Unbeknownst to heroine Nora, the secretary for a jewelry firm, her father is the head of a gang of jewel thieves. Posing as a nobleman, the gang boss steals a priceless diamond belt, shifting the blame to Nora's innocent sweetheart Brahms. It is up to Sexton Blake, his right-hand man Harry, and his faithful dog Pedro to retrieve the belt and expose the guilty party.