Act I – The Church of Sant'Andrea della Valle in Rome.
The curtain rises to the orchestral rumble of three crushing chords, a musical motif associated with the chief of police, Baron Scarpia. This is followed by a second motif, a restless, descending syncopated theme that relates to the pursuit of the fugitive Cesare Angelotti. He appears in the church, exhausted and trembling with fear. A Republican sympathizer, he is imprisoned by the royalists. His escape is arranged by his sister, the Marquise Attavanti, who has left him a key to the family chapel and hidden a woman's disguise there. Angelotti finds the key and hides in the chapel.
The sacristan arrives at the church. He complains about his duty to wash the brushes of the artist Mario Cavaradossi. Cavaradossi paints a portrait of Mary Magdalene. He has captured the image of the Marquise when she came to the church ostensibly to pray. Cavaradossi compares the blond and blue-eyed Magdalene (the Marquise) he painted with his beloved Floria Tosca, a more Mediterranean, dark beauty. He shares that Tosca alone possesses his heart. The sacristan departs, leaving a basket of food and wine for dinner for the Cavaradossi.
Angelotti comes out of hiding. Cavaradossi recognizes his old friend. The two reminisce about their shared political sympathies. They are interrupted by Tosca's voice from outside, "Mario!" Cavaradossi gives Angelotti the basket, and sends him into the chapel.
Tosca is deeply religious, but also obsessively jealous. She enters, irritated, filled with suspicion because she thinks she overheard Cavaradossi talking to a woman. Her jealousy is inflamed by the painting. Kavaradossi subdues her with his ardour. The lovers plan to meet at Cavaradossi's villa after Tosca's evening appearance.
After Tosca leaves, Angelotti comes out of hiding. He reveals that he plans to escape Rome. Cavaradossi gives him a key to his villa outside the city and advises him, if there is danger, to hide in the well in the garden. A cannon shot from the fortress announces that he is an escaped prisoner. Cavaradossi and Angelotti quickly leave the church.
The sacristan returns. He and the choir boys are delighted that there will be a celebration in honour of the Queen and a performance by opera singer Floria Tosca in the evening at the Palazzo Farnese. The merriment is interrupted by the unexpected arrival of Baron Scarpia and his agents from the police. Silence descends, everyone remains as if numb. Scarpia informs the sacristan that an important prisoner has just escaped and that he believes he has taken refuge in the church.
Scarpia finds a fan with the arms of the Marquis of Attavanti. Agents carry a basket of provisions out of the chapel. Scarpia puts all the findings together: not only has Angelotti taken refuge in the chapel, but Tosca's lover, the artist Cavaradossi, has not only aided the fugitive, but certainly knows his whereabouts.
Tosca returns to the church looking for Cavaradossi – but finds Scarpia and his agents. Scarpia politely flatters Tosca. He hints that the Marquise is Cavaradossi's mistress and that they have eloped together to his villa. Enraged and filled with jealousy, Tosca rushes to the artist's villa to expose the lovers, unaware that Scarpia is sending agents to follow her.
The sacred music of the Te Deum rises and gains power; people begin to fill the church. Scarpia gloats about his impending success: he will find and execute Angelotti; he will destroy Cavaradossi as well, and his ultimate victory will be the conquest of Tosca. Scarpia kneels in prayer, revealing the true passion in her soul: Tosca, mi fai, dimenticare Iddio (Tosca, you make me forget God).
Act II – Scarpia's apartment in the Palazzo Farnese.
Scarpia dines alone, and in the courtyard below Queen Carolina hosts a party. A new cantata will be sung by Floria Tosca. Scarpia instructs Sciarrone to wait for Tosca at the palace entrance and convey to her that the Baron will be expecting her after the cantata.
Spoletta reports that they have not found Angelotti in the villa. They detained Cavaradossi there and brought him to the Palazzo Farnese. While Scarpia is questioning the artist in the room, Tosca is heard singing the cantata from the courtyard below. Scarpia accuses Cavaradossi of having met the escaped prisoner Angelotti, supplied him with food and clothing, and taken him to his villa. The artist angrily denies it.
Tosca arrives and is stunned to see Cavaradossi there. She rushes to embrace him and he whispers to her that she must not say anything about what she saw in the villa. Scarpia announces that the judge is in the next room awaiting Kavaradossi's testimony. The Baron tries to manipulate Tosca, but she denies knowing Angelotti's whereabouts. The moans of the tortured Cavaradossi are heard. Scarpia again advises Tosca to save her lover by revealing the truth. Cavaradossi urges her to be brave and remain silent. Scarpia orders the torture to be intensified. Tosca cannot bear the pain of her beloved any longer. To save him, she betrays Angelotti's hiding place. Cavaradossi is brought in, wounded and bleeding. Tosca comforts her beloved with kisses and tears. Scarpia exclaims defiantly "In the well, in the garden!" He sends Spoletta there. Cavaradossi curses Tosca's betrayal.
Sciarrone rushes in with the news that Napoleon has defeated the Austrians at Marengo. Cavaradossi, weak and tortured, is overcome by a patriotic outburst at the news of victory: Vittoria! Vittoria! He curses Scarpia and all tyrants, thus revealing his revolutionary republican sympathies. Scarpia immediately sends him to prison for execution. Left alone with Tosca, the Baron hints that perhaps the two of them can save him. Tosca abruptly interrupts his game and haughtily offers to negotiate: Quanto? Il prezzo? (How much? Name the price!). Scarpia admits that he has long admired her beauty, and her anger and hatred of him have only strengthened his resolve. Tosca realizes she is powerless. Scarpia pays no heed to her pleas for mercy. But his attempts to win her over through persuasion fail, and he lashes out at her physically.
Ominous drumbeats can be heard in the distance – a signal that the gallows is being erected for the execution. Scarpia warns Tosca that there is no time. Her beloved has only an hour to live, and it is now in her power to decide his fate.
Tosca's spirit is broken. In the aria Vissi d'Arte (I lived for art), Tosca asks herself "Why has God forsaken me?" and pours out her despair in a poignant prayer.
Spoletta arrives and reports that Angelotti has been captured, but has taken poison and died. When he asks about Cavaradossi, Scarpia turns to Tosca, she now has power over her lover's fate.
Scarpia is treacherous, but Tosca is determined to save her beloved. She nods in agreement. Scarpia arranges a mock execution. Spoletta shows full understanding of Scarpia's sinister and duplicitous intentions. The Baron orders Spoletta to allow Tosca into the prison at four o'clock to tell Cavaradossi the plan. Tosca demands that he grant the lovers a pass to leave Rome. Scarpia agrees.
As he writes the pass, Tosca notices a knife on the table. She grabs the knife and hides it behind her back. Scarpia walks towards Tosca, exclaiming victoriously, Tosca finalmente mia! (Tosca, mine at last!). As he tries to embrace her, Tosca plunges the knife into his heart: 'Here is the kiss of Tosca that you so passionately longed for'. Tosca looks around frantically for the miss, finds it, and leaves cautiously.
Act III – The Fortress Sant'Angelo.
The early dawn is barely perceptible over the Eternal City – sheep bells are ringing, the morning bell is ringing from the surrounding churches. A shepherd boy sings in the distance. The jailer awaits the condemned man and the firing squad. A group of soldiers bring Cavaradossi to him, and he tells him he has only one hour to live. The artist asks if he can write a few words to his beloved.
Cavaradossi's farewell to his beloved Tosca contains memories of starry nights when they shared the bliss of love: E lucevan le stelle (The stars were shining). Cavaradossi reflects in the face of death: E non ho amato mai tanto la vita (I have never loved life so much!), and his last words are: Muoro disperato (I die in despair).
Tosca rushes excitedly to Cavaradossi and shows him the pass, triumphantly explaining how she killed Scarpia. He tenderly praises her hands that have changed their fate. Tosca explains to him that he must stage a mock execution. When they shoot, he has to fall down and play dead. After the soldiers retreat, the two will escape.
Cavaradossi was called in for a shootout. Tosca covers his ears as the officer lowers his sabre, the signal to fire. Cavaradossi falls, Tosca is proud that her lover has played his part well, and exclaims, Ecco un artiste (What an artist!). Tosca urges Cavaradossi to get up and realizes the awful truth – "Dead! Dead!" Scarpia has broken his promise, Cavaradossi is killed.
The voices of policemen are heard. Spoletta rushes to grab Tosca, who pushes him away hard. He rushes to the parapet, but before she leaps and finds her death, he cries, O Scarpia, avanti a Dio (O Scarpia, we will meet before God). The final music bursts into passionate grandeur, a thunderous return to the climactic refrain of Cavaradossi's farewell: E non ho amato mai tanto la vita (I have never loved life so much!).
