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Amateur detective Peter Cornish (William Gargan) and dimwitted police inspector Killian (Paul Hurst) combine forces to track down a blackmailer. Cornish's interest in the case is more than professional: among those being blackmailed is pretty heiress Joan Rankin (Florence Rice). The case takes on a whole new coloring when the extortionist is murdered, and Joan falls under suspicion. On the other hand, the guilty party could be Nelson the butler (Wyrley Birch), who was being indirectly victimized by the dead man. For the most part, Blackmailer is played for laughs, presumably to keep the Hays Office from complaining that the picture was too morbid.