Rome, June 1800. Cesare Angelotti, an escaped political prisoner, rushes into the church of Sant’Andrea della Valle. He hides in one of the chapels just before the painter Mario Cavaradossi arrives to work on his portrait of Mary Magdalene. The painting has been inspired by the Marchesa Attavanti, whom Cavaradossi has seen in the church but does not know. He is struck by the resemblance of the dark-haired beauty of his lover, the singer Floria Tosca, and that of the blonde Marchesa Attavanti. Angelotti, who was a member of the former Bonapartiste government, emerges from his hiding place. Cavaradossi recognizes him and promises help, then hurries him back into the chapel as Tosca is heard calling from outside. She jealously asks Cavaradossi whom he has been talking to and reminds him of their rendezvous that evening. Suddenly recognizing the Marchesa Attavanti in the painting, she accuses him of being unfaithful, but he assures her of his love. When Tosca has left, Angelotti again comes out of hiding. A cannon signals that the police have discovered the escape, and he and Cavaradossi flee to the painter’s house. The sacristan enters with choirboys who are preparing to sing in a Te Deum celebrating the recent victory against Napoleon at the Battle of Marengo. Their excitement is silenced by the arrival of Baron Scarpia, chief of the secret police, who is searching for Angelotti. When Tosca comes back looking for Cavaradossi, Scarpia shows her a fan with the Attavanti crest that he has just found. Seemingly finding her suspicions about her lover’s infidelity confirmed, Tosca bursts into tears. She vows vengeance and leaves as the church fills with worshippers. Scarpia sends his men to follow her to Cavaradossi, with whom he thinks Angelotti is hiding. While the congregation sings the Te Deum, Scarpia declares that he will bend Tosca to his will.