The Case of the Baby Sitter (1947) Tom Neal, Pamela Blake, Allen Jenkins

Jewel thieves, operating under the guises of a Duke and a Duchess, hire the Ace Detective Agency, run by Russ Ashton, to baby sit an infant they have kidnapped and are using as a blind. A rival gang of thieves dope the detective sitter, Harvard Quinlan, and make off with a stolen and valuable diamond.