ACT I: The Male and Female Chorus set the background. Rome is occupied by the Etruscan king, Tarquinius Superbus, whose son Tarquinius Sextus "treats the proud capital as if it were his whore." In a military camp outside the city, three generals, Tarquinius, Junius and Collatinus, are drinking and quarreling. The previous evening, they had ridden into the city to test the fidelity of their wives and only Collatinus' wife Lucretia had proved faithful. The unmarried Tarquinius, challenged by Junius' drunken comment that "women are chaste when they are not tempted" cries out "I'll prove her chaste" and rides swiftly to Rome. As Lucretia and her servants Bianca and Lucia, work, Lucretia longs for her absent husband. The three women are preparing for bed when Tarquinius knocks at the door. Reluctantly, Lucretia offers him hospitality and shows him to a room for the night.

ACT II: Tarquinius awakens the sleeping Lucretia with a kiss. She cries out and struggles against his advances, but he overpowers her and rapes her. The next morning Lucretia sends a flower to Collatinus with a message that it comes from a Roman harlot, but Collatinus, warned by Junius of Tarquinius' sudden departure the previous evening, has returned to Rome. Lucretia tells him what has happened and, despite of his forgiveness, she kills herself.