Cesare Angelotti stumbles into the church of Sant’ Andrea della Valle. He has just escaped from the Castel Sant’ Angelo prison, where he has been incarcerated for political reasons by Baron Scarpia, chief of the Roman police. Angelotti searches for and discovers a key which has been hidden for him by his sister, the Marchesa Attavanti. With it he admits himself to the Attavanti chapel just as the Sacristan appears. Mario Cavaradossi enters to resume work on a painting of the Mary Magdalene. He is startled by the appearance of his old friend. Angelotti. The latter explains his plight, but their conversation is interrupted by the voice of Floria Tosca.

Cavaradossi unlocks a door to admit Tosca. The two plan to meet that evening after the performance in which Tosca must sing. She notices the painting and recognizes the features of the Marchesa Attavanti. In a fit of jealousy, she accuses Cavaradossi of infidelity, but he manages to soothe her. After she leaves, Cavaradossi opens the chapel where Angelotti is hiding. As they talk, a canon shot is heard which announces Angelotti’s escape. Cavaradossi offers to guide him to a hiding place and they rush off.

Word has been received of victory over Napoleon and acolytes crowd into the church. Their excitement is stilled as Scarpia enters to search for Angelotti.

Tosca returns to tell Cavaradossi that she will be unable to meet him that evening because she must sing at the victory celebration being organized by the Queen. Scarpia, to play on her jealousy, shows her a fan with the Attavanti coat of arms which has just been found in the chapel. Enraged, Tosca exits. Scarpia signals his agent, Spoletta, to follow her. Scarpia exults in his power and vows to possess her.

As Scarpia waits in his apartment in the Farnese Palace, Spoletta enters to say that the pursuit of Tosca has yielded no clue as to the whereabouts of Angelotti. Cavaradossi is brought in to be interrogated about Angelotti, while the voice of Tosca is heard singing a cantata from the Queen’s apartments below. Scarpia has Tosca summoned to his chambers, but when she arrives Cavaradossi cautions her to be silent. In an adjoining room Cavaradossi is put to torture, and hearing his cries of agony, Tosca is driven to divulge to Scarpia Angelotti’s hiding place. Cavaradossi is brought to Tosca, but when he learns that Tosca has revealed Angelotti’s hideout, he denounces her. Cavaradossi is dragged off to his execution and Scarpia, left alone with Tosca, indicates that he is prepared to bargain for her lover’s life. He tells her that a mock execution will be staged if Tosca will give herself to him. Horrified, she agrees. While Scarpia writes the necessary safe-conduct, which will enable Tosca to leave Rome with Cavaradossi, she notices a knife on a table. Scarpia approaches to embrace her and Tosca plunges the knife into him. She retrieves the safe-conduct from his fist and escapes.

It is just before dawn when Cavaradossi is brought to the ramparts of the Castel Sant’ Angelo for his execution. He is thinking of Tosca and suddenly she appears. She tells him of the mock execution that Scarpia has authorized, and that afterwards she and Cavaradossi will leave Rome together. The firing squad enters and takes its place. Shots ring out and Cavaradossi falls. Tosca cautions him not to move until the executioners have departed. Suddenly she discovers that the bullets were real and that Cavaradossi is dead. Cries from below tell her that the body of Scarpia has been discovered and, as his agents rush to apprehend her, Tosca leaps to her death.