A political prisoner escapes from jail and takes refuge in the church where Cavaradossi is painting. Cavaradossi helps him. Tosca arrives — beautiful, jealous as ever. Then Scarpia arrives, looking for the fugitive. He uses Tosca's jealousy as a lever to track him down.

Scarpia has Cavaradossi arrested and tortured. He summons Tosca and offers her a deal: if she becomes his lover, Cavaradossi will be shot with blank cartridges — a mock execution, then both go free. Tosca accepts. She signs the safe-conduct. Scarpia moves to embrace her. Tosca picks up a knife from the table and stabs him. 'This is Tosca's kiss!' — then places two candles beside the body and leaves.

Tosca reaches Cavaradossi on the roof of the castle. She explains the plan: the execution is fake, he must fall like an actor. The firing squad shoots. Cavaradossi falls. Tosca runs to him laughing. She shakes him. He does not get up. Scarpia betrayed her: the bullets were real. At that moment the soldiers arrive. Tosca climbs onto the parapet. 'O Scarpia, before God!' — and leaps into the void.