Paul Lavond, wrongly convicted of robbing his own Parisian bank and killing a night watchman more than seventeen years previously, escapes from Devil's Island with Marcel, an old scientist who is near death. Marcel yearns for his wife Malita and a continuation of his work, while Paul lives for vengeance that he has promised against the three partners who framed him for their crime. On the night that Paul and Marcel reach Malita's home, Marcel and Malita secretly conduct an experiment on Lachna, a young serving girl who works for Malita. Previously, Malita had been able to shrink living animals into a miniature size. After performing the experiment on Lachna, the girl is reduced to the size of a little doll, whose actions are stimulated by hypnotic thought processes. Marcel dies soon after the experiment, but Malita wants to continue with her work. Paul sees the doll-like creature as a way to exact his revenge and, though Malita is more interested in science, she agrees to go with him to Paris. Paul disguises himself as Mme. Mandelip, an old woman who owns a toy shop and, through the use of drugs, captures others who are then reduced in size by Malita. When he secretly visits his mother, she tells him that, after his wife's death, his daughter Lorraine grew up in poverty and hatred of her father, whom she believed to be guilty, and now works very hard as a laundress in a wash house. More resolved than ever to enact his plan against his former partners, Paul goes to his first victim, Victor Radin, whom Paul visits disguised as Mme. Mandelip. Paul shows Radin one of the "animated" animal dolls and Radin is so impressed that he agrees to invest in Mme. Mandelip's invention. One night, when Radin visits the toy shop, he too, is turned into a doll. When Emil Coulvet learns that Radin is missing, he becomes terrified that Paul is finally getting his revenge and asks the police for help, but Paul's disguise enables him to elude capture. As Mme. Mandelip, Paul sells a doll to Coulvet's wife and, by means of thought transference, is able to instruct the doll to steal Mme. Coulvet's jewels and throw them into the street where Paul is waiting in his disguise. He next instructs the doll to inject Coulvet with a drug that causes him to become completely paralyzed. Meanwhile, Paul has met his daughter as Mme. Mandelip and learns that she is a lovely, though bitter young woman, who is engaged to a taxi driver named Toto. After Coulvet's paralysis, Charles Matin, the third partner, becomes so frightened that he calls the police to seek their assistance. The police then come to Matin's house to protect him. Unknown to them, Radin, now doll-size, has been been hidden as an ornament on Matin's Christmas tree. Under the control of Paul's thoughts, Radin slips down from the tree and sneaks under the furniture. He is about to stab Matin in the leg when the terrified man jumps to his feet and confesses all his crimes in the hope of saving himself from Paul's vengeance. With his revenge now complete, Paul wants to stop the experiments, but Malita angrily insists that they continue Marcel's work. The two argue, and Malita causes an explosion that destroys the doll shop. Unable to save Malita, Paul escapes into the night. The next day, in the ruble of the shop, the police conclude that Mme. Mandelip is, indeed, the person responsible for the devil dolls. Later, Paul hires Toto's taxi and as they drive through Paris, reveals his identity. Wanting to speak privately, the men go to the top of the Eiffel Tower, where Paul explains that he must disappear forever because of the crimes he has committed. Paul explains that his only purpose was to have the Lavond family's reputation restored and make sure that the money stolen from him will go to Lorraine and his mother. Despite Toto's urging to stay, Paul starts to leave but hesitates when Lorraine, whom Toto had forgotten was to meet him there, arrives on the tower's elevator. Paul then tells Lorraine that he escaped from Devil's Island with her father, who died. After telling her of her father's unfaltering love for her, Paul says goodbye and enters the tower elevator as Toto and Lorraine lovingly embrace. On the way down, Paul smiles as he tells the elevator operator that this has been the greatest evening of his life.