The British That Dangerous Age is based on +Autumn, a play by Margaret Kennedy and Ilya Surgutchoff. Myrna Loy heads the cast as Lady Brooke, the wife of famed barrister Sir Brian Brooke (Roger Livesay). Neglected by her husband, Lady Brooke inaugurates an affair with a younger man. Meanwhile, Monica (Peggy Cummins), Brooke's daughter by a previous marriage, enters into her own romantic entanglement. When Sir Brian falls ill, his wife comes to her senses, and the result is lasting happiness for all, especially Monica. The story is set on the isle of Capri, allowing for several restful and pleasing landscape shots. That Dangerous Age was originally released as If This Be Sin.