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To clear his police record, private detective Mike Stafford agrees to help Scotland Yard's Detective Superintendent Meredith with a murder probe. Mike infiltrates a property company used by archvillain Colonel Calderwood as a front for blackmail. Calderwood hires the private investigator to spy on Diana, the racket's secretary. Another gang member is murdered, and the others are suspect. Mike becomes infatuated with Diana. Calderwood shoots his associate, John Crewe, whereupon Mike kills Calderwood. The blackmail ring is exposed, and Diana and Mike plan to marry and leave the country, but police find diamonds in their luggage. Meredith arrests them for plotting the murders and attempting to abscond with the gang's cache of diamonds.