Alexandre Dumas, Sr. (Berry Kroeger) tells his son Alexandre Dumas, Jr. (Raymond Burr) the story of Joseph Balsamo (Orson Welles) aka Cagliostro. 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. He was rescued by some gypsies led by Gitano and swears revenge on de Montagne.
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 throughout Europe.
Things begin to go downhill when he enters a plot to substitute a young girl called Lorenza (Nancy Guild) for Queen Marie Antoinette along with gypsies Gitano (Akim Tamiroff) and Zoraida (Valentina Cortese). The plot is organised by de Montagne in association with Madame du Barry. They plan for Lorenza to impersonate the Queen and buy a frivolous necklace.
Zoraida becomes jealous of Lorenza, who has fallen under Cagliostro's power, forgetting her true love, Gilbert. Gilbert runs away with Lorenza. Cagliostro recaptures her and marries her, hypnotised.
King Louis XV dies and Marie Antoinette becomes queen. She orders Cagliostro to leave the country. Cagliostro gets Lorenza to impersonate the Queen and pretend to be in love with de Montagne. He buys the necklace and the Queen's reputation is damaged.
Zoraida brings Lorenza to Marie Antoinette and reveals Cagliostro's plot. Lorenza agrees to testify against Cagliostro, but at the trial, Cagliostro hypnotizes her into stating that she knows nothing. Cagliostro hypnotizes Gilbert into testifying against the queen.
However, Dr. Mesmer then uses the necklace to hypnotize Cagliostro into confessing. Cagliostro awakens from his trance and escapes with the hypnotized Lorenza. Gilbert pursues them and kills Cagliostro in a sword fight.
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