In 1807, Napoleon Bonaparte (played by Charles Boyer) visits Poland and at a state ball, he lays eyes on Countess Marie Walewska (Greta Garbo) and becomes immediately infatuated with her. Poland's leaders request Marie to visit Napoleon in an attempt to gain his support for Polish independence. Though hesitant, Marie agrees to the plan. She begins an affair with Napoleon and when her husband, Count Walewski (Henry Stephenson), finds out, he chooses to divorce her. Marie continues to be Napoleon's mistress and chooses to leave with him. Despite her initial affection for him, she witnesses Napoleon losing interest in the cause of Polish freedom and instead becoming increasingly enamored with the power. She dreams of marrying him, but Napoleon is married to Hapsburg princess Marie Louise in a ceremony organized by Talleyrand (Reginald Owen).
Marie bears a son to Napoleon and after his defeat in Waterloo, she takes their son to visit him. Marie and Napoleon bid their final farewells before he is exiled to Saint Helena.