Christine Villier is betrothed to Jacques Despard, a medical student. One evening she is assaulted by Manuel Le Grange, an escaped murderer, who strangles her father when the latter attempts to save her. Hearing of Villier's murder, Despard returns to Christine, but she does not tell him of the attack. The next morning Christine leaves her home. In Paris her son is born and christened Jacques Cordet. Twenty-nine years pass. Jacques Cordet, now criminal prosecutor for the State, lives with his mother, who is respected as the Widow Cordet. Jacques, the victim of a strange malady, is afflicted with severe headaches. While under their torture he acquires another personality, that of Lazard, "The Strangler." As Lazard he is the leader of a band of Apaches. Their rendezvous is The Scarlet Sun Inn. Here Lazard reads of the prosecutor's fight against the Apaches and his efforts to arrest the fiend, "The Strangler." In the personality of Lazard, he does not remember that he is Cordet, whom he hopes to outwit. Marie Sechard objects to her father taking a stranger, Manuel, to board with them. For thirty years Manuel has been lurking in Paris. Later Manuel robs Sechard and kills him. Dragging Marie with him to the Scarlet Inn, he shows her off to his Apache friends. Lazard wins her from Manuel in a fight, in which Lazard's wrist is wounded. He falls asleep, and when he awakes, he is unconscious of what his other self passed through in the night. At his office, Jacques, receiving "The Strangler's" letter, is amazed at the resemblance to his own handwriting. Jacques Despard, now a noted surgeon, entertains his medical colleagues and Jacques Cordet. Despard introduces Cordet to his daughter, Natalia. Cordet is seized with an attack of headache. Noting his illness, Despard and Natalia accompany him to his home. There Despard and Christine are startled at their recognition of each other. Jacques goes to his room, and Christine, believing him to be asleep, receives Despard. She tells her old lover of Manuel's assault on her and of the birth of her son, who was christened Jacques Cordet. Meanwhile, Jacques in the personality of Lazard, has gone to Marie. She tells him that it was Manuel who killed her father. As the days pass, Jacques, in love with Natalia, spends his idle hours at the home of Dr. Despard. Marie prepares to avenge her father's death. Following Manuel, she sees him enter a residence. It is the home of Dr. Despard. Natalia has given the family jewels to the butler to put into the safe. There is a scream. Natalia and the servants rush in to find the butler dead and the jewels gone. The police pronounce it the work of "The Strangler." Manuel is followed and captured with the jewels in his possession. Three weeks later, at the close of Manuel's trial, Cordet is seen as State Prosecutor. To hear the words that will send Manuel to his doom, Marie visits the court for the first time. She recognizes the Prosecutor as Lazard, her sweetheart. Christine visits the court on the final day. She almost swoons when she sees that he is Manuel. Condemned to die, he is led away. Jacques asks Dr. Despard for Natalia's hand, but he refuses, saying that some day he hopes Christine will tell Jacques about the barrier between him and Natalia. Days pass. Although Jacques pleads with his mother, Christine does not tell the story of his birth. The Apaches, though hating Manuel, vow vengeance upon Cordet. Among them is Jacques, in the personality of Lazard. He arranges to have a bomb placed under the Prosecutor's desk. Natalia is attacked. Her father responds to her call for help, and they find Jacques Cordet in the personality of Lazard. Brought back to consciousness, he is told that he is Lazard, "The Strangler," whom he has sought. Jacques realizes that he has unjustly convicted Manuel. Despard informs Jacques of the latter's dual personality. Jacques tells his mother that tomorrow he will take Manuel's place at the guillotine. She then confesses that Manuel, though he is Jacques' father, committed a far greater crime against him (Jacques) and herself. He agrees to let Manuel pay for his crime, while he, a murderer, must answer to the law. The next day Manuel is guillotined. The Apaches have placed a bomb under Cordet's desk. Marie comes to his office. Jacques telephones police headquarters that Manuel has just been executed for a crime which he Jacques...then the bomb explodes. The room wrecked, Jacques and Marie buried under the debris. Rescued, Marie tells Jacques and the others that she had witnessed the killing of the Despard butler, by Manuel and not Jacques. Jacques cleared, she dies. At the hospital, the pressure on Jacques' brain is removed by an operation, after which he will no longer suffer the curse of a dual personality. Despard now consents to a marriage between Natalia and Jacques.