Disgusted when the police department fails to apprehend the murderer of her guardian, Henri Du Bois, Celeste decides to track down the criminal herself. Her only clue is a cuff link dropped near the scene of the crime on which a sphinx is engraved, and with it, she wanders through Paris' tough Moulin Noir district. When she notices a young man wearing a tie pin of identical design, she cultivates his acquaintance and eventually asks him to visit her in her home. His suspicious behavior there convinces Celeste that he is the guilty party, and although she has fallen in love with him, she has him arrested, whereupon she learns that he is Du Bois' missing son, Andre. Further detective work reveals that the real murderer is Celeste's rejected suitor, Raoul Laverne. Upon his confession, Andre is released and eventually marries Celeste.