Judge Edward Martin, a criminal court magistrate, is found dead on a street corner early one morning, and the investigations of a coroner's jury definitely establish the fact that he was murdered. There is no clue, however, to the identity of the murderer, and the police are unable to discover any. Even the shrewd special detectives assigned to the case are baffled by the mystery. Henry Duval, prosecuting attorney and close friend of the murdered man, resolves to untangle the skein of conflicting evidence and locate the guilty man. In spite of the fact that he has of late suffered intensely from nervous trouble, which includes increasingly frequent periods of mental aberration, he foregoes his vacation and remains in the city to probe the mysterious killing of his friend. He is the more interested because he himself is the last person known to have seen the man alive, as he left the judge on his own doorstep late night of the murder. Duval soon learns that a sentimental friendship existed between the judge and Mrs. Lea Morrel, wife of a prosperous businessman. When he learns, too, that Morrel was aware of this acquaintance and had forbidden his wife to meet the judge, he himself immediately suspects the husband's guilt. He places detectives on the watch and soon is in possession of a strong chain of circumstantial evidence which seems to establish beyond all shadow of doubt the fact that Morrel, impelled by jealousy of his wife, was guilty of the deed. An indictment is obtained and Morrel, unable to furnish any alibi, is held for trial without bail. Among Morrel's close personal friends is a physician who has made a special study of nervous diseases and who coincidentally was with the coroner when he examined the body of the murdered man the day following the commission of the crime. He becomes convinced that the killing was done by someone during a period of mental irresponsibility and that the murderer did not know what he was doing at the time. Learning of the prosecuting attorney's mental trouble, and considering the fact that the official was with the judge on the evening of the murder, he sets himself to investigate the case in good earnest. On the day before the trial of Morrel he calls upon Duval and asks permission to question him, a request which is readily granted. Under the careful examination of the scientist, the truth is gradually brought home to Duval, that he himself is suspected of having dealt the blow that ended the life of his friend. The skilled questioning slowly causes him to remember a number of his own actions, things which he had done without even realizing at the time that he was doing them, until at last a vision suddenly flashes before his eyes; a vision of himself in the act of striking down his companion on that fatal night. The terrible knowledge that he is guilty of the very crime for which he is prosecuting Morrel strikes him like a thunderbolt; the blow is too much for his weakened nervous system, and he falls dead at the feet of his inquisitor.