Vitellia, daughter of the deposed Emperor Vitellio, wants the current ruler, Tito (Titus Flavius Savinus Vespasianus), assassinated because he does not return her love and has chosen as consort Berenice, daughter of the King of Judea. She tries to overcome the scruples of her admirer Sesto about committing murder for her sake. Sesto's friend Annio comes to fetch him for an audience with the emperor, revealing that Berenice will not be consort after all. Vitellia's ambitions for the throne revive, and she asks Sesto to delay his plan. Annio reminds Sesto of Annio's own desire to marry Sesto's sister, Servilia, and urges him to ask Tito for permission. The two men reaffirm their friendship.
Before the Capitol, the populace hails Tito, who declares he will help the survivors of the recent eruption of Vesuvius at Pompeii. Annio and Sesto learn that the emperor wishes to marry Servilia. Diplomatically, Annio assures Tito he welcomes the union. The emperor says the chief joy of power lies in the opportunity to help others. Annio tells Servilia the emperor wishes to marry her, but she reaffirms her love for Annio, and he admits he returns it.
In the imperial palace, Publio, a guard, shows Tito a list of those who have spoken disloyally. Tito is inclined to forgive them. The discussion is interrupted by Servilia, who confesses her prior commitment to Annio. Tito generously relinquishes all claim to her and leaves, followed by Servilia. Vitellia, angry again, tells Sesto now is the time to strike. He declares that her wish is his command. When Vitellia learns Tito is looking for her, she calls after Sesto to stop him, but it is too late.
In front of the Capitol, Sesto, who has set fire to the building, trembles with remorse. Annio, Servilia, Publio and Vitellia appear, voicing anxiety and confusion. Believing he has succeeded in killing the emperor, Sesto starts to confess but is silenced by Vitellia.
In the palace, Annio tells Sesto the emperor has escaped harm. When Sesto confesses his assassination attempt, Annio advises that telling Tito the truth will earn forgiveness. Vitellia rushes in, imploring Sesto to flee for both their sakes, before Publio enters and demands Sesto's sword; the man Sesto struck in the flaming Capitol was a fellow conspirator, Lentulo, who survived. Sesto is led off to a senate hearing.
In a public hall, the people are relieved to find Tito safe. When the emperor doubts his friend Sesto's disloyalty, Publio cautions against being too innocent in the face of betrayal. Sesto has confessed and been sentenced, with other conspirators, to be thrown to the lions. Annio agrees Sesto must be punished but asks Tito to consider the case compassionately. The emperor hesitates to sign the death decree until he has questioned Sesto, who is brought in. Alone with Tito, Sesto says he did not want the throne for himself, but he hesitates to implicate Vitellia. Tito, not satisfied with Sesto's explanation, orders him led to execution. Alone, Tito agonizes over his decision, then tells Publio that Sesto's fate will be made known at the arena. Addressing the gods, Tito says that if they want a stern ruler, they ought to take away his human heart. He leaves, and the distraught Vitellia enters, convinced Sesto has implicated her in the conspiracy, but Servilia and Annio beg her to save Sesto by becoming Tito's empress. Vitellia takes Tito's announcement of her as his choice as proof that Sesto did not betray her secret, and she realizes she must die rather than accept the throne at the price of Sesto's life.
At the arena, Tito is meeting with Sesto for the last time when Vitellia interrupts, declaring her guilt. The betrayed ruler almost hardens his heart before deciding to pardon the conspirators, valuing their repentance more than their fidelity.