Valentin Marquis de Sombreiul, alias Monsieur Simon, is known as the great master because he is the leader of a band of Parisian Apaches who mete out their own private justice to individuals who have violated their code in a secret tribunal known as the court of St. Simon. In an effort to cure Eugene, a young American longing for excitement, Valentin induces the young man to witness these horrors with the result that the youth is drawn into the Apache gang and sentenced to prison for one of their crimes. Later, after the master has disbanded his secret society and married Virginia Arlen, a charming girl from an aristocratic family, he discovers to his horror that the boy whose life he has ruined is his wife's brother. When Virginia learns the truth, she refuses to forgive Valentin, but after a period of separation, the two are reconciled by their child.