This Salzburg 2003 performance has English subtitles, so here (again!) is just a brief outline of of Act II (you can access sample arias/duets sung by the main characters by clicking on the highlighted/linked text):
In the palace, Annio tells Sesto that the emperor is still alive. When Sesto confesses his assassination attempt but refuses to give any reason, Annio advises him to admit everything to Tito and hope for forgiveness. Vitellia rushes in, begging Sesto to flee, but she is too late: a fellow conspirator has betrayed him, and Publio enters with soldiers to arrest him. Sesto asks Vitellia to remember his love.
The Roman people are thankful that the emperor has survived. Tito struggles to understand the conspirators’ motives and doubts Sesto’s disloyalty. Publio warns him against being too trusting. When it is announced that Sesto has confessed and been sentenced to death by the Senate, Annio asks Tito to consider the case compassionately. The emperor will not sign the death decree until he has had the chance to question Sesto himself. Alone with Tito, Sesto assures him that he did not want the throne for himself, but he hesitates to implicate Vitellia. Tito, not satisfied with this explanation, dismisses him. Sesto asks Tito to remember their friendship and is led off. The emperor signs the decree, then tears it up: he cannot become a tyrant and execute a friend. He cries out to the gods, saying that if they want a cruel ruler, they have to take away his human heart. Servilia and Annio beg Vitellia to help save Sesto. She realizes that she must confess her crime rather than accept the throne at the price of Sesto’s life.
In a public square, Tito is about to pronounce Sesto’s sentence, when Vitellia appears and admits that she alone is responsible for the assassination attempt. The bewildered emperor explains that his intention was to forgive Sesto anyway. He finally decides to pardon all the conspirators. The Roman people praise Tito for his kindness and ask the gods to grant him a long life.
Act II
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Pantygwydr!
The Art of Listening 
