"The Countess Cosel" is based on the true story of the beautiful Anna Constantia von Brockdorff, a German noblewoman who became mistress of Augustus the Strong, King of Poland and Elector of Saxony in 1704. Against the backdrop of life at Augustus' court the movie follows the arrival at Dresden of 24-year-old Anna Constantia von Brockdorff, who is an inordinately beautiful and scrupulously devout young married woman; her romance with the king and her "ten year reign" as his mistress - a role she consents to in the belief that she has won the king's love and commitment; and finally, her heartbreak, disillusion and struggle against him. The conflict between Anna and the king arises from the fact that Anna takes seriously not only the relationship, but the written promise of marriage which she manages to secure from him during their courtship. Neither the relationship nor the contract is held in much esteem by Augustus, who replaces her, as he has always done, with another mistress, as soon as she loses for him the charm of novelty. Her valiant refusal to return the marriage promissory note upon request enrages him and delivers her to her miserable fate.