An American writer living in Rome witnesses an attempted murder that is connected to an ongoing killing spree in the city, and conducts his own investigation despite himself and his girlfriend being targeted by the killer.

Rome, 1825. Bishop Rivarola and Colonel Nardoni are in charge of suppressing the liberal revolution. Shoemaker Cornacchia got the information that the liberal Filippo Spada is a spy and is going to denounce his revolutionary companions.