The opera is set in Rome in June 1800 during a time of political strife between the Bonapartists and the royalists.
Act One—The Church of Sant'Andrea della Valle
After the harsh opening chords, symbolising the villainous Baron Scarpia, the first voice we hear is that of the fugitive, Angelotti. He finds the key that his sister, the Marchioness Attavanti, has hidden and slips into the Attavanti family chapel. Next the sprightly Sacristan enters, carrying clean brushes for the painter, Mario Cavaradossi, who has been painting a portrait of the Madonna, and is surprised that the painter isn't there and that his basket of food is untouched. At the sound of the Angelus, the Sacristan kneels and prays.
Cavaradossi arrives to continue work on the portrait, whom the Sacristan recognises as the beautiful woman who has been praying each day at the shrine of the Madonna. Observing that her lovely face has inspired him, Mario takes out a miniature of his lover, Floria Tosca, and in the aria "Recondita armonia" he compares the two beauties—the dark-eyed Tosca and the blue-eyed inspiration for his Madonna. As Mario returns to his work, Angelotti emerges cautiously from the chapel. When Mario recognises his friend, he quickly locks the door of the church and offers his help, but hearing Tosca outside the locked doors, he gives Angelotti the basket of food, pushes him back into the chapel and warns him to be quiet.
Floria Tosca is a popular opera singer who is in love with Cavaradossi. Although her jealousy has been aroused by being kept waiting outside the church, and by the unknown beauty in the portrait, Tosca invites her lover to join her at her cottage that evening and they sing a passionate duet of anticipation ("Non la sospira la nostra casetta"). Mario then tells Tosca gently that she must leave him to get on with his work, but when she recognises the beauty in the portrait as the Marchioness Attavanti, she assumes the worst—that is the reason for the locked doors! In a fiery duet, Mario tries to convince her that her suspicions are groundless, and after receiving a promise of faithfulness from Mario, Tosca leaves. Angelotti then emerges from the chapel and Mario gives him the key to his villa, where he can hide safely. As Angelotti is about to leave, several cannon shots indicate that his escape has been discovered, so Mario decides to accompany Angelotti to the villa.
The Sacristan rushes in and is surprised that Mario is not there. He is followed by a noisy group of children, to whom he announces that in honor of Bonaparte's defeat, there will be a great Te Deum and Floria Tosca will sing a new cantata. The children greet this news with much shouting and singing, which stops abruptly when Baron Scarpia, Rome's much-feared chief of police, enters the church, accompanied by Spoletta and other officers. Scarpia tells the Sacristan that they are looking for a fugitive who has been traced to the church. Searching the church, Scarpia finds a fan with the Attavanti crest and recognises the marchioness as the subject of the portrait, while another officer finds the empty food basket in the chapel. Scarpia assumes that Cavaradossi, whom he suspects of Bonapartist sympathies, has helped Angelotti escape.
Tosca, seeking to reassure herself of Mario's fidelity, returns to the church, and is easily made jealous when Scarpia shows her the fan, which he has found, he says, among the painter's belongings. Tosca, recognising the Attavanti crest, tries to hide her jealous fury and runs off in anger. Scarpia orders Spoletta to follow her and report back on her movements. As the Te Deum begins, Scarpia sings a grim soliloquy, savagely exulting in the prospect of Cavaradossi's downfall and of his own conquest of Tosca.
Act Two—Farnese Palace
Scarpia is enjoying a solitary dinner, while the concert celebrating General Melas' reported victory over Bonaparte, at which Tosca is to be the soloist, begins in the courtyard outside. After ringing for Sciarrone and giving him a note summoning Tosca to join him after the concert, he recounts his pleasure in ruthlessly possessing what he desires ("Ha piu forte sapore"). Spoletta arrives, shaking with fear, to report that he and his men have searched Cavaradossi's villa and since they did not find Angelotti, they have arrested the painter. As Tosca's voice is heard soaring over the choir in the cantata, Cavaradossi is dragged before Scarpia and ruthlessly questioned about Angelotti. Cavaradossi denies everything and when Tosca bursts in and rushes to him, he warns her to keep silent. Scarpia tells Mario that a judge is waiting in the next room—actually a torture chamber—to hear his confession, and Mario is lead away. Scarpia then questions Tosca about the villa, but she insists that only Mario was there and remarks flippantly that Scarpia's questioning is useless.
Scarpia then describes the tortures Cavaradossi is facing as they speak and advises Tosca that telling the truth may save her lover further agony, but between his cries of pain, Mario forbids her to speak. Finally, Tosca is unable to keep silent and blurts out Angelotti's hiding place. Scarpia orders the torture stopped and has Mario brought into the room, where Tosca kneels at his side, sobbing. However, when Mario learns of her betrayal, he curses her. Sciarrone brings the news that in fact, rather than victory, Bonaparte has defeated General Melas, and while Mario exults, Scarpia pours out his hatred for this rebel and orders him to a cell. Tosca struggles with the guards, but they push her aside as they take Mario away.
Enjoying his triumph, Scarpia resumes his meal, and when Tosca asks his price for Cavaradossi's release, he replies in the aria "Mi dicon venal" that his price is Tosca herself. She shrinks from him in horror and as Scarpia tells her that in an hour, her lover will die, Tosca laments in the anguished aria "Vissi d'arte" that she has lived only for her art and for love, harming no one, but now God has deserted her. Defeated and desperate, she capitulates and strikes a deal with Scarpia, who instructs Spoletta to stage a fake execution, "just like we did with Count Palmieri." Scarpia also agrees that Tosca may attend the execution and tell Cavaradossi of his reprieve, and he issues a passport for them that will allow them leave the country safely. While Scarpia is writing, Tosca picks up a knife from the dinner table and when Scarpia tries to claim his prize and embrace her, she plunges the knife into his chest, whispering fiercely, "Here is Tosca's kiss! She then wrenches the passport from his hand, places lighted candles on either side of his body, removes a crucifix from the wall and places it in his chest, and leaves as the curtain falls.
Act III—The rooftop of the prison, Castel Sant'Angelo
It is daybreak before Cavaradossi's execution. A shepherd boy is heard singing in the distance, followed by church bells ringing matins. The jailer enters and Cavaradossi is brought in by military guards. He asks permission to write a farewell letter and sadly recalls his past happiness with Tosca in the aria "E lucevan le stelle". Tosca arrives carrying her safe conduct pass, and when the lovers are left alone, she tells him all that has happened ("Il tuo sange o il mio amore volea"). Tosca shows him the passport and explains the plan for the mock execution and they sing happily of their approaching freedom. When the firing squad arrives, Cavaradossi refuses a blindfold. As Tosca covers her ears, there is a burst of fire and Mario falls. After covering the body, Spoletta and the officers leave and Tosca approaches Mario, warning him not to move until she is sure they are alone. When Tosca discovers that Mario is really dead, she throws herself on his lifeless body with a desperate cry. Spoletta returns with Sciarrone and shouts angrily that Tosca will pay for Scarpia's murder, but before they can capture her, she hurls herself over the parapet, crying "Scarpia, avanti a Dio!"—"Scarpia, we meet before God!"
