Rome 1800 – Cesare Angelotti, an escaped political prisoner, hides in the chapel of the church of Saint Andrea Della Valle while A Sacristan begins his daily rounds. Mario Cavaradossi, a painter, enters next. Cavaradossi resumes painting his portrait of Mary Magdalene, whom he notes looks like Marchesa Attavanti, a beautiful woman he has seen around town. He compares the features of his painting to his lover, Floria Tosca. Angelotti recognizes his friend and political ally, Cavaradossi, and emerges from his hiding place. While talking, they hear Tosca approaching and Angelotti returns to his hiding place. When Tosca enters, she recognizes the resemblance in Cavaradossi’s painting to Marchesa Attavanti and accuses him of infidelity. Cavaradossi reassures Tosca of his love and fidelity to her, and they confirm their plans to meet later that evening. Tosca leaves and Angelotti again comes out of hiding. Angelotti and Cavaradossi hear canons fire, a sign that the police have discovered Angelotti’s escape. They hurry to leave the church.
The Sacristan brings news of the Royalist’s triumph over the invading french forces when the corrupt chief-of-police, Baron Scarpia arrives to search for Angelotti. After questioning the Sacristan, he does find a fan with the Attavanti family seal on it and an empty food basket. Scarpia realizes that Cavaradossi helped Angelotti escape. When Tosca reappears, Scarpia shows her the fan, convincing her that Cavaradossi left with another woman. Tosca storms out of the church again, full of jealousy. Scarpia sends his spies to follow her and capture Angelotti as the congretation arrives to sing praises for Bonaparte’s defeat.
That evening at the Farnese Palace while the queen hosts a celebratory gala, Scarpia anticipates the joy of executing his manipulative plan. His spy, Spoletta, arrives, bringing in Cavaradossi. Spoletta was unable to find Angelotti, so he brought Cavaradossi in for questioning instead. Scarpia sends for Tosca, she sees Cavaradossi as he is being dragged away to be tortured. When she arrives, Scarpia says that he will torture Cavaradossi unless Tosca discloses Angelotti’s location. At first she refrains, but when Cavaradossi’s screams get louder, she concedes and tells Scarpia where Angelotti is located. Cavaradossi is released but he is furious at Tosca for her betrayal. A soldier in Scarpia’s political party runs in to notify Scarpia that they have just won an important battle in the war. Cavaradossi yells in support of liberty, defying Scarpia who sentences him to be executed.
Tosca petitions Scarpia to release him. Scarpia bargains with Tosca that he will release Cavaradossi if she gives herself to him. Tosca fights off Scarpia’s advances, but after Spoletta enters with news of Angelotti’s suicide, Tosca agrees. Scarpia states that he must still conduct a mock execution of Cavaradossi. As soon as Scarpia writes and signs the documentation to free Cavaradossi, Tosca grabs a knife, stabs Scarpia, and runs from the scene.
Before dawn on the roof of the Castel Sant’Angelo, Cavaradossi awaits his imminent execution. He bribes the jailer to deliver a farewell letter to Tosca. Suddenly, Tosca appears and explains the plan for their escape — and they begin to dream about their future together in freedom. Tosca tells Cavaradossi he must feign death to ensure the execution looks authentic. After the execution, Tosca runs to Cavaradossi only to find he is dead. Scarpia lied about the mock execution. Spoletta rushes in to arrest Tosca for the murder of Scarpia, but she hurls herself over the edge of the roof to her death vowing to meet Scarpia in front of God.