Afternoon, June 17, 1800. Church of Sant’Andrea della Valle, Rome. Cesare Angelotti, an escaped political prisoner, enters the church and hides in his family chapel. A painter, Mario Cavaradossi, comes in to work on his portrait of the Madonna, inspired by a woman he observed praying in church the day before—the Marchesa Attavanti, Angelotti’s sister. While the old Sacristan performs his duties, Cavaradossi compares the blonde Madonna he is painting to the raven beauty of his lover, the singer Floria Tosca.
When the Sacristan leaves, Angelotti emerges from hiding and asks for Cavaradossi’s help. They are interrupted by the entrance of Tosca, who has come to see her lover. Cavaradossi, who has to prevent Tosca from seeing Angelotti, quiets her jealous fears. The two men hear a cannon shot—a signal that Angelotti’s escape has been discovered. They flee to hide Angelotti at Cavaradossi’s villa.
The Sacristan and the acolytes enter, excited about late-breaking news of Napoleon’s defeat. Scarpia, chief of police, enters, having tracked Angelotti to the church. Tosca returns to find her lover gone. Scarpia sows seeds of jealousy in Tosca, suggesting that her lover is having an affair with the Marchesa. Tosca rushes out to find the painter. Scarpia sends his minions to follow her, hoping that Tosca will lead them to Cavaradossi and Angelotti. He anticipates punishing the rebels and enjoying Tosca for himself. A joyful crowd fills the church to celebrate Napoleon’s defeat.
Scarpia, at dinner in his headquarters in the Farnese Palace, sends for Tosca. When his men bring in Cavaradossi, Scarpia interrogates the painter about Angelotti’s whereabouts. He gets nowhere, and sends him off to the torture chamber adjacent to his office. Tosca appears, and Scarpia makes her listen to Cavaradossi’s torture. She can’t bear it and tells Scarpia where Angelotti is hiding. Scarpia’s men rush off to capture Angelotti. A message arrives, correcting the earlier news: Napoleon has won, not lost, at Marengo. Cavaradossi, weakened by torture, explodes with a defiant cry of victory. His words are treason, and Scarpia has him dragged off to prison.
Tosca is left to bargain with Scarpia for her lover’s life. Scarpia suggests that he will exchange Cavaradossi’s life for a moment of Tosca’s love. Tosca fights him off and prays to God for help. She yields to Scarpia’s demands in exchange for a mock execution and a safe conduct pass for her and Cavaradossi. As he is writing the pass, Tosca’s eyes light upon a dinner knife, and she plans an alternative solution to her dilemma.
Before dawn, atop the Castel Sant’Angelo, Cavaradossi remembers a night of bliss in his garden with Tosca. Tosca arrives at the prison with the safe conduct letter and tells Cavaradossi how she dealt with Scarpia. She explains to him that they must proceed with the farce of a fake execution, and she instructs him on how to fall and how to pretend to die.. But when the soldiers come for the firing squad, Scarpia has the last laugh. Tosca takes her quarrel with Scarpia before a higher judge.