Rome, in the year 1800. Angelotti, former consul of the Roman republic imprisoned by Baron Scarpia, stumbles disheveled into the church of Sant’Andrea della Valle. Having just escaped from the Castel Sant’Angelo prison, he seeks and finally finds a hidden key to a private chapel into which he slips just as the Sacristan enters. The Sacristan had expected to find the painter Mario Cavaradossi working at his easel, and a moment after the Angelus is rung Cavaradossi enters. He uncovers his painting, a Mary Magdalen with blue eyes and golden hair. The Sacristan is scandalized when he recognizes the portrait as that of a lady who has been praying in the church frequently of late.

A lunch basket stands by the easel, and the painter tells the Sacristan he may have the lunch. As the Sacristan goes off licking his lips in anticipation of a good meal, Angelotti comes out of hiding. He recognizes the painter as a political sympathizer and explains his plight. Cavaradossi offers to help. At the sound of a voice outside, Cavaradossi presses his lunch basket on the famished Angelotti and helps him back to his hiding place.

Cavaradossi unlocks the door to admit Tosca. The two arrange a tryst for that night at his villa. Just then Tosca catches sight of the painting and recognizes the features of the Marchese Attavanti. Jealously she accuses the painter of infidelity, but Cavaradossi manages to soothe her.

Tosca gone, Cavaradossi opens the chapel where Angelotti is hiding and offers to guide the fugitive to his villa, where there is a perfect hideout in an old well in the garden.

Word has been received of a victory over Napoleon, and the acolytes crowd into the church to celebrate a joyful Te Deum. Their excitement is hushed as Scarpia enters, on the trail of Angelotti. His agents find a fan bearing the Attavanti coat of arms, and Scarpia recognizes it as belonging to Angelotti’s sister. When the empty lunch basket is also discovered, Scarpia deduces that Cavaradossi has aided the prisoner’s escape.

Tosca returns to tell Cavaradossi she cannot meet him that night because she must sing at the victory celebration. Scarpia greets her. How fine it is, he says, that she comes to church to pray, not like some wantons-pointing to the painting-to meet a lover. Showing her the fan with its tell-tale crest, he is pleased to see her fly into a rage. After escorting Tosca out of the church, Scarpia orders his agent to follow her.

The evening of the same day in Scarpia’s apartment in the Farnese Palace. Music drifts up from below where Tosca is to sing for the Queen’s guests. Scarpia sends a letter to be given the singer on her arrival. Spoletta enters to report that Tosca was followed to a villa but that Angelotti was not found there. Scarpia’s anger is somewhat assuaged by news that Cavaradossi has been taken.

As Tosca’s voice soars from the floor below, Cavaradossi is questioned and denies any knowledge of Angelotti’s escape. Alarmed at Scarpia’s note, Tosca rushes in and embraces the painter, who warns her under his breath to disclose nothing. Cavaradossi is taken away, and Tosca resists Scarpia’s questions. At last he tells her that his agents are torturing her lover, and a groan is heard from the next room, striking horror in Tosca. In answer to her plea for mercy, Scarpia demands to know Angelotti’s hiding place. When Cavaradossi screams with pain, Tosca can bear it no longer and tells Scarpia that Angelotti is hidden in the well at the villa. The weakened Cavaradossi is carried back in and confronted with Tosca’s confession, but after a defiant outburst he is condemned to death.

Left alone with Tosca, Scarpia bargains for her lover’s life. She is horrified to hear that she is the price demanded. Finally, Tosca consents. Quickly Scarpia explains that a mock execution will be staged and directs Spoletta to attend to its details; ‘just as in the case of Palmiere,’ he says, with deadly significance. As Scarpia writes out a safe conduct permit for the two lovers to leave Rome, Tosca’s eyes fall on a knife which she surreptitiously takes. His writing completed, Scarpia approaches his victim with arms outstretched. As he embraces her, Tosca stabs him, exulting in his death agonies.

At dawn the following day. Cavaradossi is led from his cell to the place of execution. He is granted permission to write a letter, but is overcome with despair at the thought of never seeing Tosca again. Just then, she is led into the prison. Flying into her lover’s arms, she shows him the safe conduct. Cavaradossi can hardly believe her story. She explains the necessity of the mock execution, and the lovers laugh about the ruse. When the firing squad arrives, Cavaradossi refuses a blindfold. Orders are given, the soldiers take aim, the officer’s sword falls, shots ring out. Cavaradossi falls to the ground.

“How well he acts,” boasts Tosca. She cautions him not to move until the firing squad has left. As soon as the two are alone, she tells him he may rise. He remains motionless. She raises the cloth covering him. Scarpia has tricked them – Cavaradossi is dead. Cries from outside tell her that Scarpia’s murder has been discovered. As his agents rush in, Tosca flings herself from the parapet with a defiant cry.