The French revolutionary armies, led by Napoleon Bonaparte, are at war with the rest of Europe. Rome has briefly been a Republic under French protection but has now fallen to the Allied forces. Cesare Angelotti, former Republican Consul, has escaped from prison. He takes refuge in the Church of Sant’Andrea della Valle, where his sister, the Marchesa Attavanti, has hidden a key to her husband’s family chapel, where he hides. The artist Mario Cavaradossi returns to the church, where he is working on a fresco that depicts Mary Magdalene. He tells the shocked sacristan that the face of the Magdalene is that of the mysterious woman who has been praying near the chapel—in fact, Angelotti’s sister. Angelotti emerges once the sacristan has gone. He recognizes the painter and begs for his help. Cavaradossi’s lover, the singer Floria Tosca, calls from outside, and Angelotti hides again. The jealous Tosca suspects that Cavaradossi has been with another woman in the church, but he calms her fears. Turning to go, she spots his painting and immediately recognizes the Marchesa Attavanti. She accuses him of being unfaithful, but he again assures her of his love. When Tosca has left, a cannon signals that the police have discoveredAngelotti’s escape, and he and Cavaradossi flee to the painter’s villa. The sacristan excitedly enters to tell the church choir that the Allies have won a great victory against the French at Marengo in northern Italy. As they celebrate, Baron Scarpia, chief of Rome’s secret police, arrives looking for Angelotti. His agents search the chapel, and he discovers the Marchesa Attavanti’s fan. Scarpia recognizes her in Cavaradossi’s portrait, and when Tosca returns, he uses the fan to trick her into believing that Cavaradossi is unfaithful after all. She vows to have vengeance and leaves as the church fills with worshipers. Scarpia sends his men to follow her; he knows that she will lead them to Cavaradossi and Angelotti. While the congregation intones the Te Deum, Scarpia declares that he will bend Tosca to his will.
Dining that evening in his chambers at the Palazzo Farnese, Scarpia anticipates the pleasure of having Tosca in his power; the diva will be singing that night in the Palazzo at a royal gala to celebrate the Allied victory. The agent Spoletta has broken into Cavaradossi’s villa and found no trace of Angelotti, but he has arrested Cavaradossi and brought him to the Palazzo. Scarpia interrogates the defiant painter and sends for Tosca. When she arrives, Cavaradossi whispers an urgent plea for her to keep his secret before Scarpia’s agents lead him into another room. Scarpia begins to question Tosca. At first, she keeps her nerve, but when Scarpia tells her that Cavaradossi is being tortured in the next room, her courage fails her. Unable to bear Cavaradossi’s screams, Tosca reveals Angelotti’s hiding place. The agents bring in Cavaradossi, who is badly hurt and hardly conscious. Scarpia cruelly reveals her betrayal, and Cavaradossi angrily curses her. Suddenly, word arrives that the news from Marengo was false; Bonaparte has won the battle. Cavaradossi shouts out his defiance of tyranny, and Scarpia orders him to be executed. Once alone with Tosca, Scarpia calmly suggests that he would let Cavaradossi go free if she’d give herself to him. She refuses, but Scarpia becomes more insistent, trapping her with his power over Cavaradossi’s life. Despairing, she prays to God for help. Spoletta bursts in; rather than be captured, Angelotti has killed himself. Tosca, now forced to give in or lose her lover, agrees to Scarpia’s proposition. Scarpia orders Spoletta to prepare for a mock execution of Cavaradossi, after which he is to be freed. Tosca demands that Scarpia write her a passage of safe conduct. Once done, he embraces Tosca, but she seizes a knife from the dining table and stabs him. Before fleeing with the safe-conduct pass, she performs funeral rites over Scarpia’s body.
At dawn, Cavaradossi awaits execution on the platform of Castel Sant’Angelo. He bribes the jailer to deliver a farewell letter to Tosca and then, overcome with emotion, gives in to his despair. Tosca appears and explains what has happened. The two imagine their future in freedom. As the execution squad arrives, Tosca implores Cavaradossi to fake his death convincingly, then watches from a distance. The soldiers fire and depart. When Cavaradossi doesn’t move, Tosca realizes that the execution was real and Scarpia has betrayed her. As Scarpia’s men rush in to arrest her, she cries out that she will meet Scarpia before God and leaps from the battlements.