Rome in 1800 is in political chaos. Cesare Angelotti, a former consul of the short-lived Roman Republic, has escaped imprisonment, and seeks refuge in the church of Sant'Andrea della Valle. The painter and republican sympathizer Mario Cavaradossi promises to hide him at his country villa. Baron Scarpia, Rome's tyrannical Chief of Police, suspects that Cavaradossi has helped Angelotti to escape. He persuades the painter's opera singer lover Floria Tosca, whom he himself desires, that Cavaradossi has betrayed her. She leaves to confront Cavaradossi at his villa and Scarpia orders his men to follow her, in the hope they'll find Angelotti.

Puccini had toyed with setting Victorien Sardou's 'hit' play La Tosca (written for the great actress Sarah Bernhardt) from the early 1890s, but only began serious work following the premiere of La bohème in 1896. He employed Bohème's gifted librettists Giuseppe Giacosa and Luigi Illica to streamline Sardou's complicated plot and give depth to his rather stereotypical characters. The opera's premiere at Rome's Teatro Costanzi on 14 January 1900 was indifferently received by the critics, many of whom disliked Tosca's violence. However, the public loved it and it has remained hugely popular.