Jack Gordon, a private detective, becomes interested in a mysterious murder, which has baffled the police. Warren, a wealthy widower, whose hobby is music, has been shot to death in his home one night while playing Chopin's "Seventh Prelude." Only two people are in the house at the time, Alice Warren, his pretty daughter, and James Cummings, her uncle. Warren's will contains a provision appointing Cummings executor of the Warren fortune and guardian of Alice, she being under age. To this Alice objects absolutely, and Cummings can do nothing. The girl now develops a strange mental hallucination, apparently. Each night, at the very hour of her father's death, she hears the "Seventh Prelude" being played in the great mansion, and declares it to be her father calling to her. Cummings consults a psychologist, who begins to treat the girl for an apparent form of insanity. It is at this time that Gordon becomes interested in the case, secures all the facts of the murder from Alice, and declares his intention of saving her. Learning from the lawyer that Alice has objected to Cummings as her guardian and executor of her fortune. Gordon thoroughly examines the Warren mansion and decides on immediate action. With his assistant Hall, he goes to the Warren home that night and in the most extraordinary manner overcomes Cummings in a desperate hand-to-hand struggle, discloses a hidden phonograph in a closet with a piano rendition record of Chopin's "Seventh Prelude" and unravels the mystery of Warren's death. Cummings himself had committed the murder, and balked by Alice's objection to him as executor and guardian, had shrewdly planned to have her declared insane by playing the phonograph record each night at the hour of Warren's death, thus overruling her objection in court and gaining free access to her fortune. Luckily Gordon's handling of the case brings the murderer to justice, Cummings confesses and Alice is saved.