Act I.  In Rome, during the time of the Napoleonic Wars, Cesare Angelotti, an escaped political prisoner, takes refuge in the Church of San'Andrea della Valle.  There, he meets the painter Mario Cavaradossi, who is working on a portrait of Angelotti's sister, the Marchesa Attavanti, as Mary Magdalene.  As he paints he amuses himself by comparing his subject's beauty to that of his lover, the tempestuous diva Floria Tosca ("Recondita armonia").  When Angelotti recognizes the artist as a fellow revolutionary and asks for his help, Cavaradossi agrees, but decides to hide his involvement from Tosca, whom he expects to visit him in the church.  Tosca's jealousy is aroused at Cavaradossi's delay in opening the door, and at the subject of his painting, but he assures her of his love ("Non la sospiri la nostra casetta").  The two revolutionaries leave for a hiding place at Cavaradossi's villa as a cannon shot announces Angelotti's escape.  The church becomes a center of activity as the Sacristan announces that Baron Scarpia, the Chief of Police, will hold a gala celebrating the government's victory.  Scarpia and his men search the church, and suspecting that Tosca and Cavaradossi are implicated in Angelotti's escape, the Baron preys on Tosca's jealousy by suggesting that her lover has been unfaithful with the Marchesa.  Knowing that Tosca will immediately confront Cavaradossi, he orders his men to follow her.  As the Te Deum is sung, Scarpia plans to hang Angelotti, and seduce Tosca.

Act II.  In Scarpia's apartment in the Farnese Palace, the Baron anticipates the enjoyment of Tosca's favors ("Ha più forte sapore").  The spy Spoletta has arrested Cavaradossi, but has not found Angelotti.  When Tosca is summoned to the Palace after singing in the victory gala, Scarpia's questioning and Cavaradossi's screams as he is tortured unnerve her, and she reveals where Angelotti has hidden.  Cavaradossi is enraged, but shouts defiance as news is brought that Napoleon has won the Battle of Marengo, a major defeat for Scarpia's side.  Cavaradossi is dragged off to prison.  Scarpia then makes his offer — Tosca's body for her lover's life.  In despair, Tosca laments that her devotion to God and her art cannot save her from this fate ("Vissi d'arte").  Scarpia explains that he will order a mock execution, and prepares a safe-conduct pass for Tosca and Cavaradossi.  As Scarpia tries to embrace her, Tosca stabs him, places candles at his head and a crucifix on his chest, and leaves. 

Act III.  At the Castel Sant'Angelo just before dawn, Cavaradossi writes a last letter to Tosca ("E lucevan le stelle").  She arrives with the safe conduct pass, and tells him that she has killed Scarpia, and that the two of them can look forward to a happy future together after the fake execution.  But after the shots have been fired, Tosca realizes that the bullets were real; Scarpia, whose body has been discovered, has exacted revenge from beyond the grave.  She evades the guards and leaps from the battlements to her death, crying that she and Scarpia will meet before God.