Joseph Balsamo (Orson Welles), a poor French gypsy with a tragic past, is taken under the wing of Dr. Mesmer (Charles Goldner), who teaches him the secrets of hypnosis. While Balsamo promises the doctor that he will use the power only to help others, he instead abuses his gift in order to steal money and forge himself a powerful reputation. Using his newfound wealth and power, he hatches a devious plan to replace French queen Marie Antoinette (Nancy Guild) with a young woman, Lorenza.