The fugitive patriot Cesare Angelotti, escaped from Castel Sant'Angelo, is sheltered by painter Mario Cavaradossi.

Baron Scarpia, regent of the Pontifical Police, hunts him down by luring Floria Tosca, Cavaradossi's mistress, into thinking that her lover is cheating on her.

The woman, trailed by Scarpia, heads for Cavaradossi's house, hoping to catch him in the act, but finds him in Angelotti's company.

The letter of safe conduct written, Scarpia is then stabbed by Tosca, who then bolts off to Castel Sant'Angelo to be reunited with her lover.

Life in Rome continues seemingly unchanged, with the clergy oblivious to the new times looming, and the changes about to overtake the world.