Strange crimes committed by a hypnotized woman are solved by a French detective.

Soon after the murder of Danton, an agent of the French secret police, beautiful Parisian flower vendor Eugenie Dorain is kidnapped by minions of General Hans Moloff, a half-Russian, half-Chinese madman. Then, after determining that only Danton and Anton, Eugenie's foster father, were informed of the identity of Eugenie's Russian parents, Moloff murders Anton. When police detective François St. Cyr later discovers Anton's body, he suspects Eugenie's boyfriend, Leon Renault, a notorious pickpocket, but on a hunch, decides to use him as a guide into the Parisian underworld. While St. Cyr has his undercover agents comb the city for Eugenie, Moloff cables the Grand Duke Maxim, the brother of the slain Czar Nicholas II, that he has located Princess Anastasia, Nicholas' only surviving child, then hypnotizes Eugenie and brainwashes her to believe that she is the princess. Soon after, St. Cyr hears a report that the supposed princess bears a resemblance to Eugenie and dispatches Leon to Moloff's chateau to investigate. As Moloff tries to convince Maxim that the entranced Eugenie is Anastasia, Leon breaks into the heavily guarded chateau and stumbles into Moloff's basement, which is filled with sinister looking scientific tools. Although he is chased off before finding Eugenie, Leon conveys his suspicions to St. Cyr, who is then called to investigate an automobile accident near the chateau. At the accident site, St. Cyr finds Maxim's body and a document signed by Maxim declaring Moloff's Anastasia as Nicholas II's rightful heir. After St. Cyr's men determine that Maxim's death was caused by an elaborately produced optical illusion, St. Cyr tries to question Eugenie but is turned away from the chateau. As Moloff prepares to flee with Eugenie, Leon returns to the chateau to rescue her but is caught by the general. Seconds before her death at the hands of Moloff, St. Cyr and the police break into the chateau and force Moloff to kill himself using his own instruments of death.