Male and Female Chorus are seated on either side of the stage, reading from books. They explain the events against which the action of the opera takes place.
By murder and treachery, the Etruscan Tarquinius Superbus has become King of Rome and rules in a reign of terror. His son Tarquinius Sextus leads a Roman army against the Greeks, to distract attention from conditions in Rome, "and treats the proud city as if it were his whore."
A curtain rises to reveal the army camp outside Rome with the generals' tent where Collatinus, Junius and Tarquinius are drinking. It is night. They discuss the events of the previous night when some of generals had ridden back to Rome to check up on their wives and found most of them unfaithful, including the wife of Junius. Collatinus tells them they were foolish to go and the others comment that he has no need to worry since his wife Lucretia was the only one found virtuously at home.
Tarquinius taunts Junius with the faithlessness of his wife and they quarrel acrimoniously. Collatinus calms them and Tarquinius proposes a toast to the chaste Lucretia. Junius bursts out of the tent, unable to bear the comparison between Lucretia and his wife. Male Chorus comments, remarking how lucky Collatinus is to have chosen a virtuous wife.
Collatinus comes out and reproaches Junius for venting his rage against his own wife in bitterness against Lucretia and, and Junius apologises. When Tarquinius joins them and resumes his taunting of Junius, Collatinus reproves him and makes peace between the two. Satisfied with his work he retires to bed.
Tarquinius reveals his obsession with Lucretia, claiming that she is "as chaste as she is beautiful" and declaring that he will "prove Lucretia chaste." Junius leaves him with an admonition that he will not dare. As Tarqunius paces up and down Male Chorus utters his thoughts:
Tarquinius does not dare,?When Tarquinius does not desire;?But I am Prince of Rome?And Lucretia's eyes my Empire.
Tarquinius breaks in in his own voice calling for his horse.
Male Chrous relates Tarquinius' ride to Rome.
SCENE 2: The hall of Lucretia's home
Lucretia, her nurse Bianca and her maid Lucia are sewing and spinning. Joined by Female Chorus, they sing of a woman's fate.
Lucretia starts up, thinking she hears a knock and hoping it might be a messenger from Collatinus - even though, as Bianca reminds her, she has already had two letters from him that day. But there was no one at the door. Lucretia laments her separation from her husband.
It is late and they begin folding linen while Female Chorus reflects on the lot of women. They prepare to go to bed while Male and Female Chorus alternate, she describing the sleeping city and he the approach of Tarquinius.
(In the scene that follows they continue to describe what is happening while the characters mime the actions described, only occasionally uttering lines of their own.)
Tarquinius knocks at the door and is admitted. He claims hospitality, alleging his horse is lame. Lucretia is unable to refuse him but the two servants mutter suspiciously. Bianca in particular fears danger and resents his presence. They all bid one another good night and depart for bed.
Male and Female Chorus read from their books a commentary on the character of the Etruscans and are interrupted by the off-stage mutterings of Collatinus, Junius, Bianca and Lucia about the corruption of Rome under their rule. The Choruses discard their books and explain that they are viewing the action from the standpoint of the present day and according to Christian standards.
The curtain rises, showing Lucretia asleep with a candle beside her. Tarquinius approaches steathily, described by Male Chorus. He wakes Lucretia with a kiss and she reveals her deep-seated fear of him: "In the forest of my dreams you have always been the Tiger."
He begs for her love and when she refuses siezes her forcibly. She struggles free and he threatens her with his sword, the Choruses commenting the while. As Tarquinius beats out the candle with his sword the curtain falls quickly.
The Choruses lament the triumph of sin and pray for relief.
SCENE 2: The hall of Lucretia's house the next morning
Bianca and Lucia greet the lovely day and collect flowers to adorn the house. They leave the orchids for Lucretia to arrange as they are Collatinus' favorite flowers. Bianca says that she thinks she heard Tarquinius gallop away before dawn. She is glad that Lucretia is still asleep because she is so often wakeful, pining for Collatinus.
Lucretia appears and tells Bianca that she slept "as heavily as death." When Bianca gives her the orchids her calm is suddenly shattered and she cries out in horror at them. She tells Lucia to send a message to Collatinus with one of the flowers. The message is wild and distraught: "Give him this orchid and tell him a Roman harlot sent it."
Lucretia then takes the flowers but twines them into a wreath. Bianca is perturbed at this, but Lucretia reminds her how she had taught herself to weave garlands when she was young. Lucretia goes and Bianca remembers her in happy former days. When Lucia comes back Bianca tells her the message must not be sent, even though Lucretia had ordered it: "Sometimes a good servant should forget an order."
But it is too late: Collatinus has already come, accompanied by Junius who had heard Tarquinius leave the camp and return at dawn and warned Collatinus "too late", as Collatinus tells him as Lucretia approaches dressed in purple mourning garments.
Collatinus and Lucretia speak of the intensity of their love: "To love as we loved was to live on the edge of tragedy." Lucretia tells him how Tarquinius had raped her and thus violated the integrity of their love. Collatinus tries to comfort her with the argument that there was no shame since she had not consented, but she feels too deeply shamed and stabs herself.
Collatinus, Junius, Bianca and Lucia utter a lament in which Male and Female Chorus join. The Choruses too conclude by contemplating the tragedy, the suffering and the pain in Christian terms and proclaiming their faith in Christ's forgiveness.