The Devil and Miss Jones (1941)

Department store owner J.P. Merrick finds that several of his employees are unionizing to get more money and better working conditions. In order to find out who the organizers are, he gets a job at the store as a shoe salesman. Not realizing his true identity, he's befriended by Mary Jones and Joe O'Brien, the two ringleaders, and Elizabeth Ellis, a charming older woman with whom he develops a romance.