Alexandre Dumas, Sr. (Berry Kroeger) tells his son Alexander Dumas, Jr. (Raymond Burr) the story of Joseph Balsamo (Orson Welles). Through flashbacks we learn that Balsamo was a French gypsy boy who endured much hardship. He was tortured under the command of Viscount de Montagne (Stephen Bekassy) and his parents were ordered to hang. Some years later, he learns the secrets of hypnosis from Dr. Mesmer (Charles Goldner).. Ignoring the doctor's advice that he use his powers for healing, he exploits his new talent to the full, gaining wealth, fame and prestige. After changing his name to Count Cagliostro, he becomes famous all over Europe. Things begin to go downhill when he enters a plot to substitute a young girl called Lorenza (Nancy Guild) for the French queen Marie Antoinette along with gypsies Gitano (Akim Tamiroff) and Zoraida (Valentina Cortese)..