Lucio Silla, the tyrannical dictator of Rome, has killed his political rival Marius Gaius and has banished Cecilio, the senator engaged to marry Gaius’s daughter, Giunia.

Cecilio has secretly returned to Rome. He learns from his friend and ally Cinna that Silla has declared him dead and proposes to marry Giunia himself. Cinna advises Cecilio to wait for Giunia in the heroes’ burial vault, where she goes daily to grieve for her father. Meanwhile, Silla asks his sister Celia to help him overcome Giunia’s contempt and win her hand. Celia advises him to use patient love, rather than force. When Giunia responds to Silla’s overtures with hatred and scorn, Silla decides he must overcome his affection and condemn her. Giunia goes to the heroes’ burial vault to lament her dead father and her banished lover, but Cecilio interrupts her and the two lovers are reunited.

Silla continues to plot to marry Giunia. Employing a ruse to ensure that she will become his bride, he agrees to let Celia marry Cinna at the Capitol that very day. Cecilio arrives, intent on killing Silla, but is dissuaded. Cinna tells Giunia that the dictator is about to ask the Senate for permission to wed her. Celia advises Giunia to accept her fate but she decides to throw herself on the mercy of the Senate. At the Capitol, the populace happily anticipates Silla’s marriage. He asks the Senate for its approval. Giunia threatens to stab herself, but Cecilio interrupts the action and is arrested. Seeing the lovers console each other makes a deep impression on Silla.

Cinna pleads with Celia to ask her brother for mercy. Giunia visits Cecilio in prison for a last farewell. She plans to die with him, since she will not yield to Silla’s desires in order to save his life. At the Capitol, Silla condemns Cecilio, but then suddenly reveals his decision to let Cecilio live and marry Giunia. Cinna confesses to his own part in the plot to assassinate Silla, but the dictator forgives him as well and blesses his marriage to Celia. Silla renounces the crown, proclaiming that he is granting freedom to his kingdom and will live out his life as one of its citizens.