July 17, 1800, three days after the Battle of Marengo.

ACT I — The Church of Sant’Andrea della Valle.

We see where an artist has been at work on a painting in the church. Angelotti dashes in, out of breath from his escape from the Castel Sant’Angelo where he was held as a political prisoner. He finds the key to one of the chapels which his sister has left for him and hides as the Sacristan approaches. The Sacristan is surprised to find the painter’s lunch but no painter. As he kneels to say the Angelus, the artist, Mario Cavaradossi, arrives. The Sacristan gazes at the painting and sees that Mary Magdalene of the painting resembles a lady he has seen praying in the church. Mario confesses he used the stranger as a model. He compares her blond, blue-eyed beauty with that of his dark-haired love, the famous singer, Floria Tosca. After reminding Cavaradossi to lock up, the Sacristan leaves. Angelotti reappears and is delighted to recognize the painter as an old friend. Tosca calls through the locked door as Cavaradossi gives Angelotti his lunch basket and  tells him to hide once more. Mario admits Tosca who is suspicious at the locked door and whispered voices. She thinks it was another woman with the painter. When he tries to kiss her, she insists upon saying a prayer first. When a preoccupied Cavaradossi does not react to her plans for the evening, the annoyed Tosca describes how romantic it will be. Then she notices the painting, recognizes the blue-eyed model, and once more suspects Mario. He finally convinces Tosca that he loves only her and her dark eyes. He teases her about her jealousy. As she leaves, she tells him to paint the Magdalene’s eyes dark. The mysterious woman in the painting is actually Angelotti’s sister and, as well as the key, she has left women’s clothes and a fan for him to use as a disguise. Cavaradossi gives Angelotti the key to his own house and tells him he can hide there, in the well if necessary. When a cannon sounds, announcing the escape of a prisoner has been discovered, the friends rush out together. The Sacristan enters calling to the choir boys; Napolean has been defeated in Northern Italy, and a celebration is being planned – the Te Deum to be sung at a special service. They are interrupted by Baron Scarpia, the chief of police, who orders his men to search the church for the escaped Angelotti. Finding the empty food basket and the embroidered fan belonging to Angelotti’s sister, they guess that the prisoner has been there. When Scarpia learns the name of the painter, he sees a sinister way to force Tosca to his will. Once more we hear Tosca calling for her Mario. Scarpia shows her the fan, confirming her former suspicions. As people arrive for the service of thanksgiving, Tosca laments her lover’s supposed unfaithfulness, and Scarpia feigns pity. When the jealous singer runs off to find Mario, Scarpia instructs his henchman, Spoletta, to follow her. Scarpia gloats “Va, Tosca” (Go, Tosca), and cries, “Tosca, mi fai dimenticare Dio!” (Tosca, you make me forget God!)”

ACT II — That evening, Scarpia’s quarters in the Palazzo Farnese.

Elsewhere in the palace Tosca is singing as Scarpia dines in his chambers, thinking of his capture of Cavaradossi and Angelotti, and his conquest of Tosca. Spoletta arrives; he has captured Mario but cannot find Angelotti. From offstage we hear Tosca singing. Cavaradossi is brought in and questioned but denies any knowledge of the escaped prisoner. Tosca enters. Mario embraces her, whispering that she should say nothing about what she saw at his house. Scarpia has the artist tortured and Tosca, in distress, reveals that Angelotti is hiding in the well in Mario’s garden. Scarpia sends Spoletta to find the escapee. When a bleeding and faint Cavaradossi is brought in, Tosca lies, telling him she has said nothing. Suddenly news is brought that Napoleon has defeated the Allies at the battle of Marengo after all, a blow for Scarpia’s side. Cavaradossi exults in the victory. In spite of Tosca’s pleas, Scarpia orders Mario executed for his defiance.  Left alone with the police chief, Tosca asks the price for Mario’s freedom. Scarpia doesn’t want money — he wants Tosca. When news is brought that Angelotti had killed himself rather than surrender, Tosca gives in. Scarpia makes a deal with Tosca. He changes the sentence from hanging to the firing squad and let’s Tosca know that it will be a fake execution. He then instructs Spoletta to execute Cavaradossi’s sentence “as we did with Palmieri”. Tosca asks for a safe-conduct to leave Rome with Mario. As Scarpia writes, Tosca takes a knife from the table. When he moves to embrace her, she stabs him crying, “Questo è il bacio di Tosca!” (This is Tosca’s kiss!). Scarpia dies, and she takes the safe-conduct from his hand exulting, “E avanti a lui tremava tutta Roma!” (And before him all Rome trembled). Now a murderer, Tosca lights candles and lays a crucifix on Scarpia’s body before she leaves to tell Mario.

ACT III — An upper courtyard at the Castel Sant’Angelo.

It is near dawn. A shepherd is heard singing a sad love song. Cavaradossi is led to a cell and asks for paper to write a last letter. He starts to write but is overcome by his memories. Tosca is brought in, shows Mario the safe-conduct, and describes the murder of Scarpia. Cavaradossi wonders that such a gentle creature could perform such a terrible deed. Tosca explains the execution plan and instructs him on how to pretend to ‘die’ . In a passionate duet, they sing of their future together. The firing squad arrives, performs the duty and departs. However, Scarpia has had his revenge from the grave. The execution was real. Hope is crushed as Tosca realizes Mario is dead and the alarm is sounded with the news of Scarpia’s murder. Men rush to capture Tosca who climbs on the parapet, exclaims, “O Scarpia, avanti a Dio!” (Oh Scarpia, we will meet before God) and leaps to her death.