That Lady in Ermine tells two parallel stories, both taking place in the small Mittel-European duchy of Bergamo, but one set in the 19th century and the other in the 16th. In 1861, the Countess Angelina (Betty Grable) and her newlywed husband, Baron Mario, have just entered the bridal chamber when a Hungarian army regiment under the command of Colonel Teglash (Douglas Fairbanks, Jr.) invades. Mario flees the castle, leaving Angelina to face the invaders, just as her 16th --century ancestor Francesca (also Betty Grable) was forced to protect her turf under similar circumstances. Colonel Teglash is struck by a portrait of Francesca (the lady in ermine of the title), and the resemblance between her and Angelina makes it difficult for him to rule with his customary iron fist. Meanwhile, Francesca haunts the dreams of both Angelina and Teglash. She appears to advise Angelina to pretend interest in the colonel and then kill him, much as she herself did so many years ago. At the same time, Teglash dreams that Francesca/Angelina overcomes the desire to kill him and falls in love with him.