The Lady Attavanti had left a woman's clothes for her brother, to wear as a disguise. He takes them up and turns to go, when the report of a cannon tells him that his flight from the fortress is discovered. With sudden resolution Cavaradossi decides to accompany the fugitive, to help him to escape from his terrible enemy.
In the next scene acolytes, scholars and singers enter the church tumultuously. They have heard that Napoleon has been defeated and all are shouting and laughing, when Scarpia, the chief of the Police enters in search of the fugitive. Turning to the sacristan he demands to be shown the chapel of the Attavanti, which to the amazement of the sacristan is found open. It is empty, but Scarpia finds a fan, on which he perceives the arms of the Attavanti, then he sees the picture and hears that Tosca's lover, Cavaradossi has painted it. The basket with food is also found, empty. During the discussion that ensues, Tosca enters, much astonished to find Scarpia here instead of her lover. The chief of the police awakens her jealousy by showing her the fan, which he pretends to have found on the scaffolding. Tosca, recognizing the arms of the Attavanti is goaded almost to madness by the wily Scarpia. When she departs three spies are ordered to follow her.
The second act takes place in Scarpia's luxurious apartments in an upper story of the Farnese palace.
Scarpia is expecting Tosca, who is to sing this evening at the Queen's festival. He has decided to take her for his Mistress, and to put her lover to death as well as Angelotti, as soon as he has got hold of both. Spoletta, a police-agent informs his chief, that he followed Tosca to a solitary villa which she left again, alone, very soon after she had entered it.
Forcing his way into the villa he had only found the painter Cavaradossi whom he had at once arrested and brought to the palace.
Cavaradossi who is now brought in, denies resolutely any knowledge of the escaped prisoner. When Tosca enters he embraces her, whispering into her ear not to betray anything she had witnessed in his villa.
Meanwhile Scarpia has called for Roberts, the executioner, and Mario is led into the torture chamber that adjoins Scarpia's apartment. Scarpia vainly questions Tosca about her visit to the villa; she assures him, that she found her lover alone. Then she hears her lover's groans, which are growing more fearful, the torture under Scarpia's directions being applied with more and more violence. In the intervals Mario however entreats Tosca to be silent, but at last she can bear no more, and gasps "In the well, in the garden." Scarpia at once gives a signal to stop the torture and Mario is carried in fainting and covered with blood. When he comes to himself he hears Scarpia say to Spoletta "In the well, in the garden," and thereby finds out that Tosca has betrayed the unfortunate prisoner. While he turns from her in bitter grief and indignation, Sciarrone enters and announces in the greatest consternation, that the news of victory have proved false, Napoleon having beaten the Italian army at Marengo. Mario exults in the defeat of his enemy, but the latter turns to him with an evil smile and orders the gendarmes to take him away to his death. Tosca tries to follow him, but Scarpia detains her. Remaining alone with him, she offers him all her treasures and at last kneels to him imploring him to save her lover. But the villain only shows her the scaffold which is being erected on the square below, swearing that he will only save her lover if she will be his. Tosca turns shuddering from him. Spoletta now enters to announce that Angelotti being found and taken has killed himself; and that Mario is ready for death.
Now at last Tosca yields, Scarpia promising to liberate her lover at the price of her honour. He suggests however that Mario must be supposed dead, and that a farce must be acted, in which the prisoner is to pretend to fall dead while only blank cartridges will be used for firing. Tosca begs to be allowed to warn him herself and Scarpia consents and orders Spoletta to accompany her to the prison at 4 o'clock in the morning, after having given the spy private instructions to have Mario really shot after all. Spoletta retires and Scarpia approaches Tosca to claim his reward. But she stops him, asking for a safe conduct for herself and her lover. While Scarpia is writing it Tosca seizes a knife from the table while leaning against it and hides the weapon behind her back. Scarpia seals the passport, then opening his arms he says: "Now Tosca, mine at last." But he staggers back with an awful scream; Tosca has suddenly plunged the knife deep into his breast. Before he can call for help, death overtakes him, and Tosca after having taken the passport from the clenched fist of the dead man turns to fly.
The third act takes place on the platform of the castle Sant' Angelo.
The gaoler informs Mario Cavaradossi that he may ask for a last favour having only one hour to live and the captive begs to be allowed to send a last letter of farewell to his fiancée. The gaoler assents, and Mario sits down to write, but soon the sweet recollections of the past overcome him. Tosca finds him in bitter tears, which soon give way to joy, when she shows him her passport, granting a free pass to Tosca and to the Chevalier who will accompany her.