The opera opens at night.  The scene is laid in a street near the Lateran Church in Rome.  Orsini, a Roman nobleman, and his friends are attempting to abduct Irene, the sister of Rienzi, a Papal notary.  They are disturbed by the entrance of Colonna, another Roman noble, and his adherents.  The two ruffians quarrel over the unfortunate girl; their followers eagerly join in the fray; and in a moment, as it seems, the quiet street is alive with the cliquetis of steel and the flash of sword-blades.  Adriano, Colonna’s son, loves Irene, and when he discovers who the trembling victim of patrician lust really is, he hastens to protect her.  The tumult soon attracts a crowd to the spot.  Last comes Rienzi, indignant at the insult offered to his sister, and bent upon revenge.  Adriano, torn by conflicting emotions, decides to throw in his lot with Rienzi, and the act ends with the appointment of the latter to the post of Tribune—–­ he refuses the title of King—­and the marshalling of the plebeians against the recreant aristocracy.  The arms of the people carry the day, and in the second act the nobles appear at the Capitol to sue for pardon.  Rienzi, though warned of their treachery by Adriano, accepts their promise of submission.  During the festivities which celebrate the reconciliation Orsini attempts to assassinate Rienzi, who is only saved by the steel breastplate which he wears beneath his robes.  For this outrage the nobles are condemned to death.  Adriano begs for his father’s life, and Rienzi weakly relents, and grants his prayer on condition of the nobles taking an oath of submission.

In the third act the struggle between the nobles and the people advances another stage.  The nobles have once more broken their oath, and are drawn up in battle array at the gates of Rome.  Rienzi marshals his forces and prepares to march forth against them.  In vain Adriano pleads once more for pardon.  The fortune of war goes in favour of the plebeians.  The nobles are routed, Colonna is slain, and the scene closes as Adriano vows vengeance over his father’s body upon his murderer.

In the fourth act the tide has turned against Rienzi.  The citizens suspect him of treachery to their cause.  Adriano joins the ranks of malcontents, and does all in his power to fire them to vengeance.  Rienzi appears, and is at once surrounded by the conspirators, but in a speech of noble patriotism he convinces them of their mistakes, and wins them once more to allegiance.  Suddenly the doors of the Lateran Church are thrown open; the Papal Legate appears, and reads aloud the Bull of Rienzi’s excommunication.  Horror-stricken at the awful sentence, the Tribune’s friends forsake him and fly, all save Irene, who, deaf to the wild entreaties of Adriano, clings to her brother in passionate devotion.

In the fifth act, Rienzi, after a last vain attempt to arouse the patriotism of the people, seeks refuge in the Capitol, which is fired by the enraged mob.  The Tribune and Irene perish in the flames, together with Adriano, whose love for Irene proves stronger than death.