Vitellia, daughter of deposed emperor Vitellio, wants revenge against Tito and stirs up Tito's vacillating friend Sesto, who is in love with her, to act against him (duet
Come ti piace, imponi). But when she hears word that Tito has sent Berenice of Cilicia, of whom she was jealous, back to Jerusalem, Vitellia tells Sesto to delay carrying out her wishes, hoping Tito will choose her (Vitellia) as his empress (aria
Deh, se piacer mi vuoi).
Tito, however, decides to choose Sesto's sister Servilia to be his empress, and orders Annio (Sesto's friend) to bear the message to Servilia (aria
Del pių sublime soglio). Since Annio and Servilia, unbeknownst to Tito, are in love, this news is very unwelcome to both (duet
Ah, perdona al primo affetto). Servilia decides to tell Tito the truth but also says that if Tito still insists on marrying her, she will obey. Tito thanks the gods for Servilia's truthfulness and immediately forswears the idea of coming between her and Annio (aria
Ah, se fosse intorno al trono).
In the meantime, however, Vitellia has heard the news about Tito's interest in Servilia and is again boiling with jealousy. She urges Sesto to assassinate Tito. He agrees, singing one of the opera's most famous arias
(Parto, parto, ma tu, ben mio with basset clarinet obbligato). Almost as soon as he leaves, Annio and the guard Publio arrive to escort Vitellia to Tito, who has now chosen her as his empress. She is torn with feelings of guilt and worry over what she has sent Sesto to do.
Sesto, meanwhile, is at the Capitol wrestling with his conscience (recitativo
Oh Dei, che smania č questa), as he and his accomplices go about to burn it down. The other characters (except Tito) enter severally and react with horror to the burning Capitol. Sesto reenters and announces that he saw Tito slain, but Vitellia stops him from incriminating himself as the assassin. The others lament Tito in a slow, mournful conclusion to act 1.
The act begins with Annio telling Sesto that Emperor Tito is in fact alive and has just been seen; in the smoke and chaos, Sesto mistook another for Tito. Sesto wants to leave Rome, but Annio persuades him not to (aria
Torna di Tito a lato). Soon Publio arrives to arrest Sesto, bearing the news that it was one of Sesto's co-conspirators who dressed himself in Tito's robes and was stabbed, though not mortally, by Sesto. The Senate tries Sesto as Tito waits impatiently, sure that his friend will be exonerated; Publio expresses his doubts (aria
Tardi s'avvede d'un tradimento) and leaves for the Senate. Annio begs Tito to show clemency towards his friend (aria
Tu fosti tradito). Publio returns and accounces that Sesto has been found guilty and an anguished Tito must sign Sesto's death sentence.
He decides to send for Sesto first, attempting to obtain further details about the plot. Sesto takes all the guilt on himself and says he deserves death (rondo
Deh, per questo istante solo), so Tito tells him he shall have it and sends him away. But after an extended internal struggle, Tito tears up the execution warrant for Sesto and determines that, if the world wishes to accuse him (Tito) of anything, it can charge him with showing too much mercy rather than with having a vengeful heart (aria
Vitellia at this time is torn by guilt, but Servilia warns her that tears alone will not save Sesto (aria
S'altro che lagrime). Vitellia finally decides to confess all to Tito, giving up her hopes of empire (rondo
Non pių di fiori with basset horn obbligato). In the amphitheatre, the condemned (including Sesto) are waiting to be thrown to the wild beasts. Tito is about to show mercy when Vitellia offers her confession as the instigator of Sesto's plot. Though shocked, the emperor includes her in the general clemency he offers (recitativo accompagnato
Ma che giorno e mai questo?). The opera concludes with all the subjects praising the extreme generosity of Tito, while he himself asks that the gods cut short his days when he ceases to care for the good of Rome.