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.