Mr. Armstrong, a banker, returns to his home in his motorcar driven by Ashton, his chauffeur, taking with him a large sum of money. His daughter, Edna, has a sweetheart, Walter Hoskins, who has been forbidden the house and ordered to cease his attentions to her. Hoskins gains entrance to the house by climbing the lattice work on the piazza and the act is seen by Ashton. Armstrong, asleep in his den, is murdered, the perpetrator being indistinct in the semi-darkness. King, an expert detective, is called into the case. Suspicion points to Hoskins, as it is known that he has had a wordy war of words with the banker. The chauffeur tells his story of seeing the young man enter the house in a clandestine manner and he is arrested and charged with the crime. Edna believes implicitly in his innocence and implores King to clear him. The sleuth makes a minute search of the surroundings, examining the person of the dead man carefully. He finds thumbprints on the collar worn by Mr. Armstrong and makes his deductions from that fact. The young man is visited in prison by his sweetheart and told to be of good cheer. King makes a thorough examination of the premises and visits the garage, examining the tools, etc. He is seen working in his laboratory, studying the thumbprint under a powerful glass and then determines to try an experiment. Placing powder on his wrist he has an interview with Ashton, shaking hands with the driver, pressing his thumb firmly on the powder. Returning home, the print is examined and he is satisfied. He visits the room in which the banker was killed and summons Ashton. Dramatically the detective goes through the actions of the murderer and, grasping the hand of the chauffeur, points to the thumb and accuses him of the crime, producing the wrench which was used to inflict the fatal blow. The chauffeur is terrified and, with shaking knees, confesses the murder. Hoskins is released from prison and is warmly greeted by his sweetheart. Together they seek out King and thank him for his interest in the case.