La clemenza di Tito

La clemenza di Tito is an opera by Wolfgang Amadeus Mozart. The text of the opera was written by Caterino Mazzola. It is based on an earlier libretto by Pietro Metastasio.
The story of the opera
Act 1
Vitellia, is the daughter of Vitellius who has lost his position of emperor. Vitellius wants revenge against Titus. Vitellia asks Titus's friend Sextus, who is in love with her, to act against him. But when she hears that Titus has sent Berenice of Cilicia, of whom she was jealous, back to Jerusalem, Vitellia tells Sextus to wait a while before doing what she had asked, hoping Titus will choose her (Vitellia) as his empress.
Titus, however, decides to choose Sextus's sister Servilia to be his empress, and tells Annius (Sextus's friend) to take the message to Servilia. Annius and Servilia are in love, although Titus does not know this, so neither of them like this news. Servilia decides to tell Titus the truth but also says that if Titus still insists on marrying her, she will obey. Titus thanks the gods for Servilia's truthfulness and immediately promises to give up the idea of coming between her and Annius.
In the meantime, however, Vitellia has heard the news about Titus's interest in Servilia and is again very jealous. She urges Sextus to kill Titus. He agrees, singing one of the opera's most famous arias, "Parto, parto." Almost as soon as he leaves, Annius and the guard Publius arrive to take Vitellia to Titus, who has now chosen her as his empress. She is torn with feelings of guilt and worry over what she has sent Sextus to do.
Sextus, meanwhile, is at the Capitol, which he and his friends are going to burn down. Sextus cannot think what is the right thing to do. The other characters (except Titus) enter and are horrified to see the burning Capitol. Sextus comes in again and says that he saw Titus being killed, but Vitellia stops him giving away the fact that he was the killer. The others lament Titus in a slow, sad end to Act I.
Act 2
Begins with Annius telling Sextus that Emperor Titus is in fact alive and has just been seen. In the smoke and chaos, Sextus thought he has seen Titus, but it was someone else. Soon Publius arrives to arrest Sextus, bringing the news that it was one of Sextus' friends who dressed himself in Titus's robes and was stabbed by Sextus, but he did not die. The Senate take a court case against Sextus as Titus waits impatiently, sure that his friend will be found not guilty; but the Senate finds him guilty, and a terribly sad Titus must sign Sextus' death papers.
He decides to send for Sextus first, trying to get further details about the plot. Sextus takes all the guilt on himself and says he deserves death, so Titus tells him he shall have death, and sends him away. But after a lot of painful thinking, Titus tears up the execution papers for Sextus and decides that, if the world wants to accuse him (Titus) of anything, then people can say that he allowed too much mercy, insteady of saying that he always wanted revenge.
Vitellia feels terrible about what has happened and decides to tell all the truth to Titus, giving up her hopes of empire. In the amphitheater, the condemned (including Sextus) are waiting to be thrown to the wild animals. Titus is about to show mercy when Vitellia explains that she started Sextus's plot. Though shocked, the emperor includes her in the general clemency (forgiveness) he offers. The opera finishes with all the people praising the kindness of Titus, while he himself asks the gods to end his life if he ever stops caring for the good of Rome.
History
In July 1791 Mozart was asked to write an opera seria. He had been asked by Domenico Guardasoni who lived in Prague and who had the job of finding someone to compose an opera for the celebrations of the coronation of Leopold II, Holy Roman Emperor as King of Bohemia. The ceremony would take place on September 6, so Mozart had to work quickly.
Guardasoni had asked Antonio Salieri to write the opera, but Salieri was too busy, so he then asked Mozart. Mozart was glad to accept, because he was paid twice as much as when he wrote an opera for Vienna.