On the run from secret police and political enemies, the activist Angelotti rushes into the Church of Sant’ Andrea della Valle, hiding as the comical old Sacristan busies himself. Soon the dashing painter Cavaradossi enters to work on his portrait of Mary Magdalene based on a likeness of Angelotti’s sister, the Marchesa Attavanti. Cavaradossi compares her dark beauty to that of his fairer lover, the charismatic diva Floria Tosca (“Recondita armonia”). Angelotti emerges and is given food by his friend Cavaradossi, who rushes him back into hiding as Tosca calls out. She jealously questions Cavaradossi before praying, then reminds him they are to meet that evening.Once she leaves, he signals Angelotti to emerge from the chapel and the two friends, learning that the police have discovered Angelotti’s escape, flee to Mario’s villa.

Meanwhile, the Sacristan returns with choirboys who are to sing a Te Deum that day. Their excitement is cut short by Baron Scarpia, feared chief of the secret police, searching for Angelotti. When Tosca reappears, Scarpia plays to her jealousy, revealing a fan with the Attavanti crest. She swears vengeance on Cavaradossi and storms out as the church fills with worshipers. Scarpia orders his men to follow her to Angelotti’s hideout; then, with the chanting chorus in procession behind him, Scarpia fantasizes aloud about sexually entrapping Tosca. His reverie continues until he suddenly remembers he is in church (“Va,Tosca!”).

From his opulent office in the Palazzo Farnese, Scarpia plans how he will manipulate Tosca to serve his political and sexual ambitions. Scarpia’s henchman Spoletta, having failed to locate Angelotti, brings in Cavaradossi; Scarpia questions him while Tosca can be heard singing a cantata elsewhere in the palace. She enters as her lover is being removed for more forceful interrogation. Unable to endure his screams, she divulges Angelotti’s hiding place and the battered Cavaradossi is carried back in; realizing that Tosca has “talked,” he denounces her. But when an officer interrupts to announce that Napoleon has defeated Scarpia’s allies in the Battle of Marengo, Cavaradossi’s bitterness turns to exultant cries of “Vittoria!” and he is dragged off to prison.

Finally alone with Tosca, Scarpia flatters her beauty and suggests that her lover’s life and freedom are in her hands. Tosca fends off his advances, turning to God in her despair (“Vissi d’arte”). Spoletta interrupts the standoff to announce that Angelotti, faced with capture, has killed himself. Cornered and without options, Tosca relents to Scarpia. He pretends to order a mock execution and sends Spoletta out, then jots a safe-conduct for Cavaradossi and Tosca. But the desperate, resourceful Tosca, kills him and removes the safe-conduct from his grasp as he dies. Placing candles at his head and a crucifix on his chest, she leaves.

The lone voice of a shepherd boy greets the dawn as Mario awaits execution at the Castel Sant’Angelo. He bribes the jailer to slip a farewell note to Tosca. Gloom overwhelms him as he writes (“E lucevan le stelle”) until Tosca rushes in, excitedly recounting her escape from Scarpia. Mario caresses the hands that killed for his sake (“O dolci mani”) and the two hail their future. As the firing squad appears, Tosca — ever the star — advises Mario on how to fake his death convincingly. The soldiers fire and depart. Tosca urges Mario to hurry, but when he remains motionless, she sees that Scarpia has outmaneuvered her one last time: the bullets were real. As Spoletta rushes in to arrest Tosca, she cries out to Scarpia to meet her before God, then leaps to her death from the castle’s parapet.