Lieutenant Joseph Petrosino, an Italian-American police detective, is assigned to a murder case involving the Black Hand Society. Soon afterwards, a ragpicker is sent to murder banker Antonio Lorenzo, who has threatened to expose the criminals' blackmail schemes. Lieutenant Petrosino ingeniously traces the crime to the ragpicker but is knocked unconscious in the process. Later, two Black Hand members flee the country, pursued to Sicily by Lieutenant Petrosino, who impresses the local police with his heroic methods of pursuit. Eventually, however, Petrosino is killed by Black Hand assassins in Palermo. His body is returned to the United States, where he is proclaimed a martyr of law and order.