Unfaithfully Yours is a remake of the 1948 Preston Sturges film of the same name. In the film, Claude Eastman (Dudley Moore) is a symphony conductor who has recently married a much younger woman, Daniella (Nastassja Kinski). While on a trip, Claude asks his friend Norman Robbins (Albert Brooks) to keep an eye on Daniella while he is out of town. Norman misunderstands Claude and ends up hiring a private investigator, Keller (Richard B. Schull), to follow Daniella. Keller ends up believing that Daniella is having an affair with a violinist in Claude's symphony. The violinist, Maxmillian Stein (Armand Assante) is known for being a ladies man so Claude instantly believes that the private investigator's report is true. Full of rage and paranoia, he plots to murder his wife for cheating on him. However, is Daniella actually cheating on Claude or is she up to something else? And will Claude kill her before he can find out?