How to Save a Marriage and Ruin Your Life

David Sloane (Dean Martin) suspects that his married friend, Harry (Eli Wallach), may be fooling around on his wife, so he intends to steal his mistress away from him. David assumes Harry is going after his secretary, Carol (Stella Stevens), and he quickly charms her into a relationship. Problem solved, David thinks, until he discovers that he assumed wrong and that Harry has actually been having an affair with his beautiful neighbor, Muriel (Anne Jackson).