The story concerns the Roman dictator Lucio Silla (Lucius Sulla) who lusts after Giunia, the daughter of his enemy Gaius Marius. Giunia, on the other hand, loves the exiled senator Cecilio.

Act 1
Scene 1: A secluded spot on the banks of the Tiber'

The exiled Senator Cecilio meets his friend Cinna who tells him his betrothed Giunia mourns his death, a lie by the dictator Silla so that he can win her for himself. Cinna advises Cecilio to meet Giunia by the tomb of her father (murdered hero Gaius Marius). Cecilio is filled with joy at the idea and Cinna shares his joy and predicts the freedom of Rome (aria: "Vieni ov' amor t' inita").

Scene 2

Cecilio excited at the prospect of meeting his betrothed sings of his love (aria: "Il tenero momento").

Scene 3: Giunia's apartments

Silla seeks the advice of his sister Celia on his approach with Giunia and she advises subtlety and kindness (aria: "Se lusinghiera speme").

Scene 4

On Silla's approach, Giunia declares her love for Cecilio and her hate for Silla, her father's enemy (aria: "Dalla sponda tenebrosa").

Scene 5

Alone, Silla, insulted, decides to behave as a tyrant (aria: "Il desìo di vendetta, e di morte").

Scene 6

Cecilio waits by the tomb for Giunia.

Scene 7

Giunia arrives (chorus and ariosa: "Fuor di queste urne dolente").

Act 2
An archway decorated with military trophies

Silla is joined by Celia to whom he tells of his plans to wed Giunia and for Celia to wed her beloved Cinna on this day.

Scene 3

Cinna restrains Cecilio who has his sword drawn trying to follow Silla, believing he has been instructed by the spirit of Gaius Marius to seek revenge. Cinna tells him to consider Giunia and his rage is controlled (aria: "Quest' improvviso trèmito").

Scene 4

Giunia consults with Cinna who suggests she accept Silla's proposal and then murder him in their wedding bed. Giunia refuses, stating that vengeance is for Heaven alone to consider. She asks Cinna to make sure that Cecilio stays hidden from danger (aria: "Ah se il crudel periglio").

Scene 5

Cinna resolves to kill Silla himself (aria: "Nel fortunato istante").

Scene 6, Hanging gardens

Silla's love for Giunia starts to bring out his compassion.

Scene 7

Giunia's hateful face angers him again and he threatens her with death but not to die alone (aria: "D' ogni pieta mi spoglio").

Scene 8

With Cecilio, Giunia worries about Silla's words and they part.

Scene 9

Celia asks Giunia to accept Silla's proposal for the sake of happiness saying she is also to be married to Cinna (aria: "Quando sugl' arsi campi").

Scene 10

Giunia ponders her wretchedness.

Scene 11, the Capitol

Silla asks the Senate and the people of Rome to reward him as a hero of Rome with the marriage to Giunia.

Scene 12

When Cecilio appears, there is confrontation (trio: "Quell' orgoglioso sdegno").

Act 3
Scene 1: Entrance to the dungeons

Cecilio has been imprisoned. Cinna and Celia has gained access and Cinna asks Celia to convince Cecilio to repent and forget his love. Cinna promises to marry Celia if she is successful, for which she is hopeful (aria: "Strider sento la procella").

Scene 2

Whilst Cecilio accepts his fate Cinna tells him not to worry, Silla's heart over his head will bring about his own downfall (aria: "De' più superbi il core").

Scene 3

Silla has allowed Giunia one last visit to Cecilio and they say their farewells (aria: "Pupille amate").

Scene 4

Giunia alone with her thoughts of Cecilio's impending death thinks of her own (aria: "Frà I pensier più funesti di morte").

Scene 5: The audience chamber

Before the Senators and the people of Rome, to everybody's surprise, Silla declares that he wishes Cecilio to live and marry Giunia. When questioned on his silence, Cinna declares his hatred of Silla and his intention of killing him. Silla issues his "punishment" to Cinna that he should marry his beloved Celia. He further declares that he will step down as dictator and restore liberty to Rome. He explains that he has seen proof that innocence and a virtuous heart is triumphant over power and glory. The people of Rome celebrate liberty and the greatness of Silla.