Act 1
The church of Sant’Andrea della Valle. Cesare Angelotti, former Consul of the Roman Republic, has escaped from the Castel Sant’Angelo, where he was being held for political reasons. He has taken refuge in the church, where his sister, the Marchesa Attavanti, has hidden woman’s clothing in the family chapel so that he can escape without being recognised. The sacristan is surprised not to find the painter Mario Cavaradossi at work. The latter, a young Roman aristocrat and a sympathiser with republican ideas, is painting a fresco in the church. By chance the beautiful Marchesa Attavanti, who has been coming to the church regularly in order to prepare her brother’s flight, has become the model for the saint the painter has undertaken to depict. Once the sacristan has left, Angelotti comes out of his hiding-place.

Mario recognises him and swears to do everything in his power to save him. They are interrupted by the arrival of Mario’s lover, Floria Tosca, a singer revered by the Romans. Angelotti disappears back to his hiding-place. When Tosca discerns the features of the Marchesa Attavanti in Mario’s portrait, her jealousy erupts, and only her lover’s ardent declarations of love succeed in dispelling her suspicions. Once Tosca has left, Mario and Angelotti are in a position to work out a plan which will allow the fugitive to avoid recapture: the painter suggests Angelotti should hide in a villa he owns. A cannon shot fired from Sant’Angelo announces that the prisoner’s escape has been discovered. The two men take flight. Under the direction of the sacristan, members of the choir school, seminarians and novices make a disorderly entrance. They have come to rehearse a Te Deum: a thanksgiving ceremony has been decreed to celebrate what is believed to be an Austrian victory over the French.

The unexpected appearance of Scarpia, the chief of police, brings a halt to the children’s expressions of joy. Accompanied by his henchmen, Spoletta and Sciarrone, Scarpia is on Angelotti’s tracks: his suspicions are confirmed by the sacristan’s confidences and the discovery of the Marchesa Attavanti’s fan. Floria Tosca has returned, but she cannot find Mario. Scarpia shows her the Marchesa’s fan and excites her jealousy. Tosca, convinced that Mario is being unfaithful to her with Attavanti, decides to surprise them at the painter’s secret villa. All Scarpia has to do to find Angelotti’s hiding-place is to have her followed. As the Cardinal’s retinue appears, Scarpia remains indifferent to the strains of the Te Deum. His thoughts are of a sensual nature and he vows to make Tosca his.  

Act 2
At the Palazzo Farnese. Preoccupied and nervous, Scarpia dines while listening to the echoes of the festivities being given in the reception rooms to celebrate "the victory” at Marengo. Tosca, to whom Scarpia has had delivered a note requesting her to call on him, is to sing a cantata composed by Paisiello in honour of the victorious Austrian general. Spoletta returns: he has arrested Mario but is unable to find Angelotti. Under questioning by Scarpia, Mario denies any part in Angelotti’s escape while Tosca’s voice comes to the fore. Tosca appears, and Scarpia has Mario led off to the torture chamber. Hearing her lover’s cries, Tosca reveals Angelotti’s hiding-place, and Scarpia sends Spoletta to capture the fugitive. Sciarrone appears suddenly and informs the police chief that it is in fact Bonaparte who has won the battle of Marengo.

Mario is jubilant. Scarpia’s henchmen take him away and their master pronounces his death sentence. However, if Tosca will agree to be his, the prisoner will be pardoned. On hearing the drum roll which precedes the execution of the condemned, Tosca capitulates. Scarpia convinces her that Cavaradossi’s execution will be a mock one. In reality, he gives Spoletta, come to announce Angelotti’s suicide, the order to have the painter shot. Before yielding, Tosca demands a safe-conduct allowing her to leave Rome with Mario. Scarpia writes out the document and signs it, but when he approaches Tosca, she stabs him.  

Act 3
A platform in the Castel Sant’Angelo. Dawn is breaking over Rome. Mario Cavaradossi is dreaming, lulled by the song of a young Roman shepherd. He awakens as the bells announce the coming of a new day. The hour of his execution approaches. Mario asks for permission to write to Tosca, but he is incapable of doing so. He remembers how happy they were together. Tosca appears suddenly and tells him how she has procured a safeconduct which will take them to freedom. Scarpia has paid the price for his crimes and is dead. The execution will take place, but the rifles will be loaded with blanks, and he must act out his death convincingly.

For a few moments the two lovers give free rein to their joy. After some final advice, Tosca leaves her lover in front of the firing squad. Mario plays his role well and Tosca is proud of her pupil, but the rifles were well and truly loaded and Mario does not get up. Scarpia’s murder is discovered. Spoletta and Sciarrone come to arrest the singer. Floria Tosca throws herself into the void.