The action takes place in Rome in June 1800
Church of Sant Andrea della Valle
Political prisoner Cesare Angelotti, who escaped from prison, enters the church and takes refuge in the chapel of his sister, the Marquise Attavanti. The artist Mario Cavaradossi appears and removes the blanket from the painting he is working on. Sacky is outraged by the similarity of the image of Mary Magdalene with the features of a stranger who recently prayed in the church. Cavaradossi compares the picture with a miniature depicting his beloved, Floria Tosca, and reflects on art and beauty (“Recondita armonia” / “Your face changes forever”). When the sacristan leaves, Angelotti tries to quietly leave the chapel, thinking that the church is empty. Seeing Cavaradossi, he finds out his friend and tells him about the escape. The artist gives the fugitive a basket of food and hides inside the chapel, as Tosca, a famous singer, enters the church.
Scarpia sends a note to Tosca for her to appear if she loves Mario: he is captured in the house. The police chief interrogates the artist. When Tosca enters, Cavaradossi has time to warn her so that she does not betray Angelotti. Cavaradossi is led away to the next room for torture. His cries make Tosca call the place where the fugitive is hiding. Mario curses her. At this moment comes the news of the victory of Napoleon. Cavaradossi appeals to Scarpia with a challenge, glorifies freedom, a republic ("Vittoria! Vittoria! L'alba vindice appar" / "Victory, victory! Anthem of freedom, sound!"). He's gone. Scarpia offers Tosca to surrender to him, in which case Mario will be saved ("Gia. Mi dicon venal" / "Yes, I love money"). If not, he will be dismissed immediately. Longing in confusion ("Vissi d'arte, vissi d'amore" / "Only sang, only loved"). Finally, she agrees, but demands that Cavaradossi be released immediately. Scarpia promises that his shooting will be done with blank ammunition. In the presence of Tosca, he presents the order to Spolette. Tosca also requires a pass for himself and Mario. While Scarpia writes, she notices a dagger on the table and decides to kill the baron as soon as he approaches her. A dagger kick knocks him off his feet. When the baron dies, Tosca takes a pass from his hand, puts a crucifix on his chest and leaves.
The prison square of Sant'Angelo
It's getting light. It rings the morning bells. Shepherd’s song is heard. A group of soldiers leads Cavaradossi to the prison tower site, where in an hour he should be shot. The artist asks to send a letter to Tosca and, having begun to write, is immersed in the memories of the past (“E lucevan le stelle” / “In the sky the stars were burning”). Tosca suddenly appears: she shows Cavaradossi the pass and tells about the murder of Scarpia. Mario kisses her hands (“O dolci mani mansuete e pure” / “O these hands, gentle hands”). Both dream of near happiness (duet “Amaro sol per te m'era il morto” / “You are with me - and I don’t need the sun”). It is time to execute. Mario falls under the shots. The longing, having run up, wants to help him to get up: the blood reveals to her the awful truth. Voices are heard: the murder of Scarpia is solved, and the police are going to arrest Tosca. But she rises to the parapet and rushes down.