Ruled by the foreigner Tarquinius Superbus and fighting off a Greek invasion, Rome has sunk into depravity. In an armed camp outside the city, Tarquinius, Collatinus and Junius are drinking together. The previous night, a group of soldiers rode home unexpectedly to Rome to check on their wives, all of whom were caught betraying their husbands, with the single exception of Collatinus' wife Lucretia. Junius, whose wife was among the faithless majority, goads young Tarquinius, the king's son, into testing Lucretia's chastity himself. The impulsive prince calls for his horse and gallops off to the city alone.

At Collatinus' house in Rome, Lucretia is patiently spinning with her servants Bianca and Lucia. She longs for her absent husband. As the women prepare for bed, there is a knock at the door: Tarquinius. Though fearful, they cannot refuse to offer the prince hospitality.

As Lucretia sleeps, Tarquinius creeps into her bedroom and awakens her with a kiss. She begs him to go, but certain that she desires him, he rapes her.

The following morning, Lucia and Bianca are glad to discover that Tarquinius has already left the house. Lucretia enters, calm but obviously devastated. She sends a messenger asking Collatinus to come home. Bianca tries to stop the messenger, but Collatinus arrives at once (accompanied by Junius). He comforts his wife lovingly, but she feels that she will never be clean again. She stabs herself and dies. All mourn. Junius plans to use this crime by the prince to spark a rebellion against the king.