Cesare Angelotti, an escaped political prisoner, runs into the church of Sant’Andrea della Valle to hide in the Attavanti family chapel. At the sound of the Angelus, the Sacristan enters to pray. He is interrupted by Mario Cavaradossi, who has come to work on his portrait of Mary Magdalene — inspired by the Marchesa Attavanti, Angelotti’s sister. Mario contrasts the beauty of the blond Marchesa with that of the woman he loves, the raven-haired singer Floria Tosca (“Recondita armonia”).
Angelotti ventures out and is recognized by Mario, who gives him food and hurries him back into the chapel as Tosca is heard calling outside. She jealously questions Mario, then prays and reminds him of their rendezvous that evening (“Non la sospiri la nostra casetta”). Recognizing the Marchesa’s likeness in the painting, she explodes with renewed suspicions, but he reassures her (“Qual’occhio al mondo”). When she has left, Mario summons Angelotti as a cannon signals that the police have discovered the escape; the two flee to Mario’s villa.
Napoleon’s army is supposed to have suffered defeat, and the Sacristan returns with choirboys who are about to sing a Te Deum. Their excitement is silenced by the entrance of Baron Scarpia, chief of the secret police, in search of Angelotti. When Tosca comes back looking for Mario, she encounters Scarpia, who shows her the Attavanti crest on a fan he has just found. Thinking Mario faithless, Tosca tearfully vows vengeance and leaves as the church resounds with the Te Deum. Scarpia has the diva trailed, scheming to get her in his power (“Va, Tosca!”).
In the Farnese Palace, Scarpia anticipates the pleasure of bending Tosca to his will (“Ha piu forte sapore”).
The spy Spoletta arrives, not having found Angelotti; to placate the baron, he brings in Mario, who is interrogated while Tosca is heard singing at a royal gala downstairs. She enters as her lover is being taken to an adjoining room for torture. Unnerved by Scarpia’s questioning and Mario’s screams, she reveals Angelotti’s hiding place.
Mario is carried in; realizing what has happened, he rages at Tosca, but the gendarme Sciarrone rushes in to announce that Napoleon has won the Battle of Marengo, a defeat for Scarpia’s side. Mario shouts his defiance (“Vittoria!”).
Tosca yields to Scarpia in exchange for her lover’s life. Fighting him off, she protest her fate to God, having dedicated her life to art and love (“Vissi d’arte”).
Spoletta interrupts: faced with capture, Angelotti has killed himself. Tosca, forced to decide, agrees to Scarpia’s proposition. The baron pretends to order a mock execution for the prisoner, after which he is to be free. Spoletta leaves.
Scarpia prepares a document of safe-conduct for the lovers. When he embraces her, Tosca stabs him with a knife, wrenches the document from his fingers and, placing candles at his head and a crucifix on his chest slips quietly out.
The voice of a shepherd is heard as church bells toll the dawn. Mario is led to the roof of Castel Sant’Angelo prison to await execution; he bribes the jailer to convey a farewell note to Tosca. Writing it, overcome with memories of love, he gives way to despair (“E lucevan le stele”).
Suddenly Tosca runs in, filled with the story of her recent adventure. Mario caresses the hands that committed the murder for his sake (“O dolci mani”), and the two hail the future.
As the firing squad appears, the diva coaches Mario on how to fake his death convincingly; the soldiers fire and depart. Tosca urges Mario to hurry, but when he fails to move, she discovers that Scarpia’s treachery has transcended the grave: the bullets were real.
Spoletta rushes in to arrest Tosca. She climbs the battlements and, crying that she will meet Scarpia before God, leaps to her death.