Act I
The political prisoner Angelotti has escaped from the Castel Sant'Angelo in Rome. He seeks refuge in a side chapel of the Church of Sant'Andrea della Valle, virtually deserted in the middle of the day, where his sister, the Marchesa Attavanti, has left him a key and clothing to be used as a disguise. Unaware of Angelotti's presence, the painter Mario Cavaradossi works on his canvas, a portrait of Mary Magdalene. The Sacristan recognizes that the artist's subject is modeled after a beautiful young woman who regularly visits the chapel.

Angelotti emerges from the side chapel and recognizes Cavaradossi as a political sympathizer. When Floria Tosca, the artist's lover, calls from outside the locked door to the church, Angelotti must hide once more. Cavaradossi gives him his own lunch basket, as Angelotti is weak from hunger and exhaustion.

Having heard voices from within the locked church, Tosca jealously assumes that Cavaradossi has been seeing another woman. Her suspicions seem to be confirmed when she recognizes Cavaradossi's Mary Magdalene as the Marchesa Attavanti, the blonde visitor to the chapel, but the painter reassures her of his fidelity and sends her on her way. A cannon shot signals the discovery of Angelotti's escape, and the two men run off to hide the fugitive in Cavaradossi's villa.

The Sacristan returns with a group of excited choristers, for word has come that invading forces have been defeated; Tosca will perform in a celebratory concert to be attended by the queen that evening. The excitement is put to an abrupt end by the arrival of the sinister police chief, Baron Scarpia. His henchmen search the suspiciously unlocked Attavanti chapel and find the Marchesa's fan (dropped by the fleeing Angelotti) as well as Cavaradossi's now-empty lunch basket. Tosca returns, and Scarpia craftily turns her against her lover, using the fan as "evidence" of his faithlessness. When she leaves, Scarpia orders his men to follow her, for he knows that the trail will lead to Angelotti. Scarpia vows that he will have Tosca-and execute her lover-at all costs.

Act II
That evening, as Scarpia dines in his residence in the Palazzo Farnese, he sends for Tosca. His henchman Spoletta reports that Tosca went straight to Cavaradossi's villa; since Angelotti was nowhere to be found on the premises, despite a thorough search, Cavaradossi was arrested instead. The concert begins and Tosca's voice is heard through the open window. Cavaradossi is brought in for questioning, but denies any knowledge of Angelotti. The performance ends, and Tosca enters Scarpia's rooms just as Cavaradossi is dragged off to the torture chamber. Unable to bear her lover's cries, she reveals Angelotti's secret hiding place to Scarpia. Scarpia offers her a bargain-her sexual favors for Cavaradossi's life. After praying for strength, Tosca agrees. Scarpia orders a faked execution for Cavaradossi, then writes out a safe conduct pass for the lovers to leave Rome. As he writes, Tosca sees a knife on the table. When Scarpia comes forward to claim her, she plunges the knife into his heart.

Act III
The next morning, on the roof of the Castel Sant'Angelo, Cavaradossi awaits his execution. He writes a farewell note to Tosca. She enters in triumph to tell him of the deal she made with Scarpia. She coaches Cavaradossi before his mock execution and the lovers sing of their hopes for the future. But when the firing squad has left, Tosca is horrified to realize that Scarpia has tricked her; the bullets were real and Cavaradossi is dead. Scarpia's men rush in to arrest her for his murder. Vowing to avenge herself before God, Tosca leaps to her death from the parapet.