The events are color-coded by act: Act I, Act II, Act III
 Angelotti is an escaped political prisoner, and using a key hidden in a water basin, is hiding out in the side chapel of Sant'Andrea della Valle in Rome.
 His friend, a painter, and his political ally, Cavaradossi, agrees to help him flee Rome.
 Cavaradossi is secretly in love with Tosca, a famous singer.
 The discovery of the escape of Angelotti is announced by a cannon shot from the Castel Sant'Angelo.
 Baron Scarpia, Chief of Police, and Spoletta, his henchman, look for Angelotti. When Tosca leaves to find her lover, Scarpia has her followed. Scarpia vows to bring Cavaradossi to the gallows and Tosca into his arms.
 Scarpia has Cavaradossi arrested, and taken away to be tortured.
 In order to save Cavaradossi, Tosca reveals Angelotti's hiding place, and makes a deal with Scarpia that he can only release Cavaradossi by faking to execute him.
 Rather than turn himself in, Angelotti commits suicide.
 Tosca insists that Scarpia pen a note of safe conduct, saying that Cavaradossi's execution will be a mock one, Tosca spots a knife, grabs it, and then plunges it into Scarpia's back.
 The mock execution turns out to be a fake mock execution (i.e., the double negatives mean he is really executed).
 When Tosca discovers this, she takes the big dirt nap.
Actual synopses can be found here and here.