Ancient Rome, ca. 80 BC. Lucio Silla, the Roman dictator, has defeated his rival Gaius Marius and seized power. He now seeks to marry Marius’s daughter Giunia. She, however, loves Cecilio, a senator whom Silla has exiled.

Act I

Cecilio returns secretly to Rome. On the banks of the Tiber, he meets his friend Cinna, who tells him that Giunia mourns his supposed death, a rumour spread by Silla. He advises Cecilio to meet Giunia at her father’s tomb.

Silla’s sister Celia meanwhile counsels him to use gentleness and guile rather than force in wooing Giunia. Yet when Silla visits Giunia, she replies that she remains faithful to her father, Marius, and to Cecilio. Rejected, Silla’s tyrannical instincts come to the fore.

Cecilio waits in hiding by the tomb, meditating on the great heroes buried there. Giunia arrives and the two lovers are reunited, their joy shadowed by danger.

Act II

Silla decides to force Giunia’s hand by decreeing their marriage before the Senate. He is urged on by his trusted advisor, the tribune Aufidio, who convinces Silla that the Senate will not dare oppose the dictator’s will. Silla tells Celia that she will marry Cinna, unaware that Celia and Cinna are already affectionate and thereby introducing power politics into their relationship.

Cecilio vows to assassinate Silla to free Giunia and avenge her father. Cinna and Giunia meet; Giunia, desperate, considers a plan to marry Silla and then poison him. Cinna resolves to kill Silla himself.

Silla warns Giunia of a cruel death not only for her but for others. She implores Cecilio to flee.

The senators, gathered on the Capitol, hail Silla, who demands Giunia’s hand in marriage; she threatens to take her own life rather than submit. When Cecilio reveals himself, he is arrested in her place. Silla rejoices as both Giunia and Cecilio are imprisoned.

Act III

Cinna visits Cecilio in prison and swears to save him. Facing what seems like certain death, Cecilio charges Cinna with the care of Giunia. Silla allows Giunia to visit Cecilio. Embracing one last time, they resolve to die together. Aufidio then arrives at the dungeon to personally escort Cecilio to his final judgment.

Cinna and Celia warn Silla that killing such noble prisoners will turn the people against him and stain his glory. Silla astonishes everyone by pardoning Cecilio and Giunia. Cinna confesses his involvement in the plot against Silla, but Silla forgives him, giving him Celia as his bride. Silla even abdicates the dictatorship, explaining that innocence and a virtuous heart triumph over power and glory.