1800, Sant’ Andrea della Valle, Rome. A man bursts into a church, where he finds a hiding place that has been prepared for him in the Attavanti family chapel. The sacristan brings something to eat for Mario Cavaradossi, who's working on a painting of Mary Magdalen. He thinks he recognises a woman, who has recently been coming to pray in the church, in the saint’s features; Cavaradossi says he is right and broods over the different manifestations of feminine beauty: the blonde Countess Attavanti (Angelotti’s sister) and his black-haired lover, the opera singer Floria Tosca. The sacristan leaves. The fugitive emergest. Cavaradossi recognises Angelotti, consul of the overthrown Roman Republic, and offers his help. Tosca arrives. Cavaradossi, not wanting to take any risks, decides not to let her know about the prisoner’s escape. He tells Angelotti to hide, and gives him the basket of food. Tosca suspects that Cavaradossi has been meeting another woman; he tries to appease her. She suggests they spend the evening together after her performance at the opera but his thoughts are elsewhere, and his lukewarm reaction rekindles her jealousy. She recognises Attavanti’s face in the painting and feels that her suspicions are justified. Cavaradossi eventually manages to calm her down and persuade her to leave; she demands that he paint Mary Magdalen’s eyes black. Cavaradossi gives Angelotti directions to a hideaway. The boom of a cannon announces that his escape has been discovered. Cavaradossi leads Angelotti to his home, to hide. The sacristan brings news about the Austrians’ victory over Napoleon and orders that preparations be made for a celebration. Scarpia enters with his thugs and is outraged about the disorder in the church. The sacristan is questioned; the secret police find Angelotti’s clothes, the empty basket and a fan belonging to Attavanti in the chapel. Suspicion falls on Cavaradossi. Tosca returns; she must cancel her date with her beloved because she is to sing at the celebrations. Scarpia feeds the flames of her jealousy by maintaining that he found the fan on the painter’s scaffolding. This provokes Tosca to hurry to Cavaradossi’s house, unwittingly leading Scarpia’s spies to where they suppose Angelotti is hiding. Scarpia indulges in fantasies of lust and power. Palazzo Farnese. Scarpia has sent for Tosca. The police informer Spoletta reports that the search for Angelotti was futile. Only Cavaradossi was arrested and brought in for interrogation, but he is giving nothing away. A celebratory cantata, with Tosca as soloist, can be heard. Scarpia orders that Cavaradossi be tortured. Tosca enters as he is being taken away; Cavaradossi begs her to say nothing. She assures Scarpia that her jealousy was unfounded. She is horrified when Scarpia tells her that her lover is being tortured. Cavaradossi reminds Tosca again to say nothing. But she can’t endure hearing his cries of pain, and tells Scarpia where Angelotti is hidden. Cavaradossi is brought in unconscious. Scarpia sends Spoletta to get Angelotti. Sciarrone brings the surprising news that the battle turned and Napoleon was victorious. With his remaining strength Cavaradossi errupts in jubilation. He is led away, his fate seems sealed. Tosca tries to negotiate a »price« for Mario’s release with Scarpia, who is known to be corrupt; but, to her horror, Scarpia demands physical remuneration instead of money. Tosca realises she has no choice. She reflects on her life and artistry. Spoletta reports that Angelotti has killed himself. Scarpia forces Tosca to make a quick decision; she agrees to the deal. Scarpia gives orders that Cavaradossi will be subjected to a sham execution by firing squad, and writes out a permit for Tosca and the painter, with which they can leave the city after the feigned execution. When Scarpia attempts to embrace her Tosca grabs a knife from the table, and stabs him. Castel Sant’ Angelo. The song of a young shepherd can be heard. Mario hands over his last possession to the prison warder in order to be allowed to write a farewell letter to Tosca. His thoughts before dying are only about her. Tosca brings news about the feigned execution and tells him how she murdered Scarpia. Both vacillate between apprehension and hope for the future. Tosca tells Mario that after the shots have been fired he must keep still and play dead until all the soldiers have left. The firing squad shoot, and Mario falls. As agreed, Spoletta prevents the sergeant from giving him a coup de grace. But, when all have gone, Mario does not get up. Tosca discovers that he really has been shot. Soldiers hurry to the platform: Scarpia’s body has been found, Tosca obviously his murderer. Before she can be seized she leaps from the castle to her death.