 Jewel thieves, operating in the guise of a Duke and a Duchess (George Meeker and Rebel Randell), hire the Ace Detective Agency, ran by Russ Ashton (Tom Neal), to "baby sit" with an infant they have kidnapped and using as a blind. A rival gang of thieves dope the "detective sitter", "Harvard" Quinlan (Allen Jenkins), and make off with a valuable diamond.