Unfaithfully Yours (1948)

Before he left for a brief European visit, symphony conductor Sir Alfred De Carter (Rex Harrison) casually asked his staid brother-in-law August (Rudy Vallee) to look out for his young wife, Daphne (Linda Darnell), during his absence. August has hired a private detective (Edgar Kennedy) to keep tabs on her. But when the private eye's report suggests Daphne might have been canoodling with his secretary (Kurt Kreuger), Sir Alfred begins to imagine how he might take his revenge.