Novelist Alexander Dumas (Berry Kroeger) tells his writer-son of Joseph Balsamo (Orson Welles), a gypsy boy in southern France who was embittered because his parents were wrongfully hanged and he himself was tortured by the order of Viscount de Montagne (Stephen Bekassy). Years later, the man, a carnival charlatan, attracts the attention of Dr. Mesmer (Charles Goldner), a pioneer in the study of hypnotism. Balsamo rejects Mesmer's plea that he use his power for healing and, instead, decides to use it to seek wealth and fame. He changes his name to Count Cagliostro (also Orson Welles), and achieves fame throughout Europe by mixing hypnotism with mysticism and showmanship. He is called to cure a girl, Lorenza (Nancy Guild), held by De Montagne, because she resembles Marie Antoinette, wife of the heir to the throne of France. Cagliostro decides to join De Montagne and Madame du Barry (Margot Grahame) in a plot to seize the power by discrediting the future Queen. Cagliostro achieves his revenge on De Montagne by persuading him to hang himself. He makes Lorenza marry him but can never make her love him.