Angelotti, a political prisoner, has escaped from the Castel Sant’Angelo and gone to the church of Sant’Andrea della Valle where his sister, the Marchioness Attavanti, has hidden the key to the family chapel. He finds the key and hides in the chapel.
A sacristan appears from the vestry and as he is reciting a prayer, Mario Cavaradossi arrives to continue the picture he is painting of Mary Magdalene. The sacristan recognises the model as the fair-haired beauty he has seen praying in the church and whose features Cavaradossi has immortalised in his painting. As he works, Cavaradossi reflects on the pale features of the woman in his painting and compares them with the dark physiognomy of Floria Tosca the singer (Recondita armonia).
The sacristan having departed, Cavaradossi becomes aware that there is someone else in the church. It is his friend Angelotti, who describes his escape and how his sister helped him. Cavaradossi realises who the woman praying in the church was. He promises to help Angelotti.
Their conversation is interrupted by the arrival of Tosca in the church. Angelotti goes back into the chapel. The jealous Tosca suspects that Cavaradossi has been unfaithful: has he been in the church with someone else? Cavaradossi pacifies her, but her jealousy and suspicion become all the stronger and in the painting she recognises the features and sky-blue eyes of the Marchioness Attavanti. Cavaradossi’s assurances of love finally convince her, but as she leaves, she asks him to give Mary Magdalene dark eyes.
Cavaradossi confides to Angelotti the way to his villa and its secret hiding place in the well. A cannon is fired from the Castel Sant’Angelo: a prisoner has escaped. Cavaradossi and Angelotti leave the church in a hurry.
The sacristan bears news of Napoleon’s defeat at the Battle of Marengo and urges the choristers to prepare a victory cantata for that night. The choristers are jubilant on hearing of the cantata and the victory celebrations. The arrival in the church of Scarpia, the Chief of Police, causes an icy silence. On interrogating the sacristan he makes the connection between Angelotti, his sister and Cavaradossi. He also recognises Mary Magdalene’s features and guesses that the painter has helped the prisoner to escape. When Tosca returns to the church to tell Cavaradossi they cannot meet that night because of the victory cantata, Scarpia shows her the Attavanti fan that has been left in the chapel. Tosca’s smouldering jealousy bursts into flames and she decides to go at once to Cavaradossi’s villa. Scarpia sends Spoletta and his men to shadow her and as the Te Deum echoes round the church, he works out a plan that would give him Tosca and lead Cavaradossi to the gallows.
Scarpia is dining in his apartment and wondering what Tosca’s shadows have discovered. Sciarrone arrives and Scarpia orders him to open the window so he can hear the cantata being performed below. He then pens a few lines to Tosca, inviting her to dine with him after the concert. Spoletta returns from Cavaradossi’s villa. Angelotti has not been found but Cavaradossi has been arrested. The painter is brought in and Scarpia interrogates him. Cavaradossi categorically denies knowing anything about Angelotti.
At that moment Tosca arrives and embraces Cavaradossi, surprised to find him here. In a whisper, Cavaradossi begs her not to mention anything that has happened.
Scarpia sends the painter for further interrogation before turning to Tosca. She, too, denies knowing anything, but then Scarpia lets on that Cavaradossi is to be tortured.
Horrified at the sound of Cavaradossi being tortured, Tosca finally discloses where Angelotti is hiding. Cavaradossi’s torment ceases and he is brought out of the torture chamber. On hearing that Tosca has betrayed him, he pushes her away. Suddenly everything is changed by the news that the Battle of Marengo was in fact won by the French. Cavaradossi’s triumphant reaction to this news annoys Scarpia, who orders him to be taken away and hanged.
When Tosca enquires the price of freeing Cavaradossi, Scarpia replies that she herself is the price. He has every intention of taking her by force but is interrupted by a sudden rattle of drums announcing the execution.
Tosca asks herself and God why she, who has lived for art and love and never done anyone any harm should have to endure all this (Vissi d’arte).
Scarpia orders Tosca to make up her mind at once. No begging or praying can make him change his mind. He wants her for himself. Spoletta appears unexpectedly with the news that Angelotti has killed himself. He also wants to know what should be done with Cavaradossi. Scarpia leaves the decision to Tosca. Tosca nods her consent but at the same time demands that her beloved be released immediately. Scarpia orders a mock execution and reminds Spoletta to observe the same procedure as on a similar previous occasion.
Tosca demands a safe-conduct pass out of Rome for herself and Cavaradossi and as Scarpia is writing it, she notices a dagger on the desk. She stealthily takes it and waits while Scarpia writes. Filled with lust, he suspects nothing and as he embraces her she plunges the dagger into his breast. After watching the death throes of the man must feared in Rome and having made sure he is dead, she takes the pass and hurries off to rescue her beloved.
An orchestral prelude heralds a new dawn. A shepherd boy’s lament mingles with the church bells calling the faithful to matins. Cavaradossi is marched to the scaffold, where a guard awaits. He asks the guard to take a last message to his beloved, promising his valuable ring as payment. He pauses in writing it as memories of their love flood his mind and make taking his leave of life painful (E lucevan le stelle).
At that moment Tosca rushes in bearing the safe-conduct pass. She tells Cavaradossi what has just happened, about Scarpia’s lust and the unexpected demise of the Chief of Police. Cavaradossi is astounded: he cannot believe it is all true. Relieved and grateful, he praises Tosca for being so brave. She tells him about the mock execution about to take place, advising him to “die” as naturally as possible. The lovers dream for a moment of a future together.
The execution squad arrives and prepare for their job. A shot is heard; Cavaradossi falls and the soldiers depart. Tosca waits for a moment before ordering him to get up, but he does not move. She rushes to him and as she raises the cloak covering him discovers to her horror that Scarpia has played a final fiendish trick: Cavaradossi is dead.
Meanwhile, Scarpia’s death has been discovered. Spoletta and Sciarrone arrive at the scaffold, Tosca climbs onto the battlements and, swearing that she will meet Scarpia before God, leaps to her death.