ACT I

The Church of Sant' Andrea della Valle

Angelotti, an escaped political prisoner, runs into the Attavanti chapel in the church of Sant' Andrea della Valle. He hides as a Sacristan is heard. Mario Cavaradossi enters to work on his portrait of Mary Magdalene – inspired by the Marchesa Attavanti (Angelotti's sister), whom he has seen but does not know. He compares the raven beauty of the singer Floria Tosca with that of the blonde Magdalene. When the Sacristan leaves, Angelotti comes out of hiding and is recognized by Cavaradossi, who gives him food and hurries him back into the chapel as Tosca is heard calling outside. She jealously questions Cavaradossi, and reminds him of their rendezvous that evening at his villa. Recognizing the Marchesa Attavanti in the painting, she explodes, but he reassures Tosca he is not having an affair with her. After she has gone, Mario and Angelotti flee to Mario's villa. Soon after, the Sacristan returns with choirboys, announcing a celebration. Baron Scarpia, chief of the secret police, searches for Angelotti. Tosca returns looking for Cavaradossi, and in comes Scarpia who shows her a fan which he has just found, decorated with the Attavanti crest. Thinking Mario faithless, Tosca tearfully vows vengeance and leaves. Scarpia sends his men to follow her and schemes to get her in his power.

ACT II

The Farnese Palace

In the Farnese Palace, Scarpia acknowledges his lust for Tosca. The spy Spoletta arrives with Mario, who is interrogated as Tosca's voice is heard singing a cantata downstairs. She enters as her lover is being taken to an adjoining room to be tortured. Unnerved by his screams, she reveals Angelotti's hiding place at the villa. Realizing what has happened, Mario turns on Tosca, but the officer Sciarrone rushes in announcing that Napoleon has won the Battle of Marengo, a defeat for Scarpia's side. Mario shouts his defiance and is dragged out. Scarpia suggests Tosca yield herself to him in exchange for her lover's life. Fighting off his embraces, she protests her fate to God, having dedicated her life to art and love. Under pressure from Scarpia, Tosca is forced to give in or see her lover killed. In front of Tosca, Scarpia orders a mock execution for the prisoner, after which he is to be freed. When Scarpia writes a safe-conduct for the lovers, Tosca snatches a knife from the table and kills him. Wrenching the document from his fingers, she slips from the room.

ACT III

The roof of Castel Sant'Angelo

Awaiting execution at the Castel Sant'Angelo, Mario bribes the jailer to take a farewell note to Tosca. Writing it, overcome with memories of love, he gives way to despair. Tosca runs in telling him that it will be a mock execution and they will be able to escape. As the firing squad appears, she coaches Mario on how to fake his death convincingly. The soldiers fire and depart. Tosca urges Mario to hurry and get up, but when he fails to move, she discovers that Scarpia's treachery has reached from beyond the grave: the bullets were real. Calling out that she and Scarpia will meet before God, Tosca leaps to her death.