ACT I -The Basilica of Sant’Andrea della Valle
Cesare Angelotti, imprisoned after being violently overthrown from leadership in the short-lived Roman Republic, rushes into the Church of Sant’Andrea della Valle. After finding a key his sister has hidden for him, he hides in their family’s private chapel. Soon, the painter Mario Cavaradossi arrives to work on his portrait of Mary Magdalene. The painting has been inspired by Angelotti’s sister, the Marchesa Attavanti, whom Cavaradossi had seen often praying in the church. Angelotti emerges from hiding and Cavaradossi recognizes him, promises to aid him, then hurries him back into the chapel as the singer Floria Tosca, his lover, calls from outside. When she comes into the church, she is suspicious that Cavaradossi has been talking to another woman, but soon they make up and make plans for a romantic rendezvous at his villa that night. Recognizing the Marchesa Attavanti in the painting, she again accuses him of being unfaithful, but he assures her of his love. After Tosca has left, Angelotti again comes out of hiding to make his own plans with Cavaradossi. A cannon signals that the police have discovered the escape, and he and Cavaradossi flee to the painter’s home.

The sacristan enters with the church choir, who are preparing to sing in a Te Deum celebrating the day’s victory against Napoleon at the Battle of Marengo. At the height of their excitement, Baron Scarpia, chief of the secret police, arrives with his team, searching for the escaped Angelotti. After finding evidence that someone was in the church helping Angelotti, Scarpia suspects that the painter, known for his pro-Napoleon stance, is the one to look for. Tosca returns, wanting to tell Cavaradossi that she would not be able to meet with him tonight, but he has disappeared. Playing the concerned acquaintance, Scarpia shows her a fan with the Attavanti crest that he found. Her suspicions about her lover’s infidelity are confirmed and Tosca is devastated. She vows vengeance and leaves as the church fills with worshippers. Scarpia sends his men to follow her, knowing that she will go directly to Cavaradossi. While the congregation piously intones the Te Deum, Scarpia declares that the prize of catching Angelotti is not nearly as enticing as the prize of bending Tosca to his will.

ACT II -Palazzo Farnese
That same night, Scarpia anticipates the pleasure of having Tosca in his power. His spy Spoletta arrives with news that he was unable to find Angelotti, but he was able to capture Cavaradossi. Scarpia interrogates the defiant painter while Tosca sings at a royal gala outside in the palace courtyard. Scarpia sends for her, and she appears just as Cavaradossi is being taken away to be tortured. Coerced by Scarpia’s questions and Cavaradossi’s screams, Tosca reveals Angelotti’s hiding place. Henchmen bring in Cavaradossi, who is badly hurt. When he realizes what has happened, he angrily confronts Tosca. Scarpia’s agent Sciarrone rushes in to announce that Napoleon has won the battle, a major defeat for Scarpia’s side. Cavaradossi shouts his defiance of tyranny, and Scarpia orders him to be executed.

Alone with Tosca, Scarpia calmly suggests that he will let Cavaradossi go free if she will give herself to him. Fighting off his advances, Tosca reflects that she has dedicated her life to art and calls on God for help. Scarpia pressures her further, but Spoletta bursts in: Faced with capture, Angelotti has killed himself. Tosca, now forced to give in or end her lover’s life, agrees to Scarpia’s proposition. Scarpia orders Spoletta to prepare a mock execution of Cavaradossi, after which he is to be freed. Tosca demands that Scarpia write her and Cavaradossi a passage of safe-conduct. After he has done so, he reaches for her, but she grabs a knife from the table and stabs him. She takes the pass and flees.

ACT III- Castel Sant’Angelo
Before dawn, Cavaradossi awaits execution on the ramparts of Castel Sant’Angelo. He bribes the jailer to deliver a farewell letter to Tosca, and then confronts his despair as his life comes to an end. Tosca appears and tells him what happened in Scarpia’s chambers. The two envision 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. Scarpia’s men rush in to arrest her; she cries out that she will meet Scarpia before God and leaps from the battlement.