The scene is laid in Rome of the fourteenth century, when the patriot, Rienzi, is leading his insurrection against the nobles. The first act represents a street riot, occasioned by the patricians, under Orsini, who have scaled Rienzi's house by a ladder and are seeking to abduct his sister, Irene.
While Irene struggles for freedom, a rival faction of patricians arrives, led by Colonna, whose son, Adriano, is in love with Irene. Adriano fights his way to her side and protects her. Then, in the midst of the disturbance, Rienzi appears and the crowd scatters.
A prelate, Cardinal Raimondo, asks Rienzi how soon he is going to begin his warfare upon the nobles, and Rienzi replies that when he hears a long trumpet-note sound across the city the hour will have come. He turns to Adriano Colonna, and fervently beseeches him to forsake his party and to join the popular cause of Roman freedom. Remembering Irene, Adriano pledges his loyalty to Rienzi.
He is then left alone with the beautiful girl, and they sink into the tender ecstasies of love, till they are roused by the ominous sound of the trumpet-call which heralds the uprising. The day dawns, and within the church the organ and chorus simultaneously break out to greet it.
Borne in by the populace, Rienzi arrives. The people seek to crown him king of Rome, but the only title he will accept is that of tribune. A great composite voice rises from the piazza, swearing vengeance on the nobles.
Rienzi's cause triumphs, and in the hall of the capitol the patricians are forced to do homage to the victor. Goaded by wounded pride, Orsini forms a conspiracy to stab Rienzi during the festivities which are in preparation.
Adriano hears the plot, and warns Rienzi, who consequently wears, when he appears at the festa, a steel breastplate.
This scene commences in an abandon of joyousness. The crowd cheers a pantomime, and knights fight in tourney.
Suddenly Orsini presses his way to Rienzi's side, and draws his knife. But Rienzi is saved by his breastplate. He sentences all the nobles to death, and the festa ends in tragedy. But Adriano pleads for his father's life, and finally Rienzi pardons all the conspirators on their oath of submission.
The third act is ushered in by alarm-bells. The nobles are again in insurrection; the people clamor for Rienzi, who appears, swearing to exterminate the faithless patricians. He goes out to victory, and presently the body of Colonna is borne past his son, Adriano, who forthwith deserts Rienzi's cause.