I Was a Shoplifter (1950)

Police detective sergeant Jeff Andrews is working on a case involving a gang of shoplifters, and he allows himself to falsely arrested as a petty thief, in order to make contact with the gang. Meanwhile, Faye Burton, a petty shoplifter and the daughter of a prominent judge, is blackmailed by the gang into joining them on the promise that they will get back a confession signed when she was caught by a department store detective, who had her sign the document rather than calling the police. It takes Jeff and Faye a lot longer to figure out who is the 'brains' behind the shoplifting gang than it does the audience, and the audience has less information than they do.