The Roman Emperor Claudio is due to return to Rome in triumph after the conquest of Britain. His wife, the Empress Agrippina, has received secret information that his ship has capsized in a storm, and the emperor is presumed dead. Driven by ambition for her son from a former marriage, Nerone, she urges him to win popular favor and be seen doing good deeds around the city. She next recruits two of Claudio’s freedmen, Pallante and Narciso, to her cause, promising sexual favors to each one in return. Nerone’s charity work goes down well in the Forum with the people of Rome, and Pallante and Narciso acclaim him as the obvious successor to Claudio, should the day come. Agrippina arrives and breaks the shocking news of Claudio’s death at sea. Pallante and Narciso immediately hail Nerone as the new Caesar, and Agrippina quickly agrees. At that moment, Claudio’s servant Lesbo rushes in with the exciting news that the emperor has been saved from the waves by the Roman officer Ottone and has landed safely on the Italian shore. Feigning joy, Agrippina welcomes Ottone, but she and her son are dumbstruck when Ottone announces that Claudio has rewarded his valor by naming him as his successor. In private, Ottone confides to Agrippina that he loves the beautiful Poppea and asks her to intercede with her on his behalf. Agrippina knows that Claudio is also pursuing Poppea and sees a way to destroy Ottone. Lesbo visits Poppea and tells her, overheard by Agrippina, that the emperor has already entered the city in secret to spend the night with her rather than his wife. Poppea admits to herself that it is Ottone she longs for, but Agrippina arrives and tells her that Ottone has betrayed her, giving her up to Claudio in his ambition to gain the throne. She advises
Poppea to take revenge by refusing Claudio’s advances, accusing the jealous and possessive Ottone of standing in their way. Poppea does as she’s told when she meets with Claudio, adding that Ottone’s pride is making him behave as defacto emperor already. Claudio promises to punish Ottone, but the approach of Agrippina interrupts his amorous pursuit of Poppea. The empress congratulates her protégée, but Poppea is torn by what she has set into motion.
Pallante and Narciso compare notes and discover that Agrippina has been playing them both. They decide to work together as events unfold. Ottone proudly anticipates the announcement of his succession to the throne as Claudio officially enters the city in triumph. But as the emperor is congratulated on his conquest, he suddenly and publicly turns on Ottone, declaring him a traitor. In turn, the others—including Agrippina and, to his astonishment, Poppea—walk away in disgust from Ottone, leaving him in uncomprehending despair. Poppea’s uneasiness about her part in Ottone’s downfall torments her. Ottone unexpectedly enters, and Poppea hides, pretending to be asleep when Ottone discovers her. She seemingly walks in her sleep, revealing Agrippina’s plot and accusing Ottone of infidelity. Ottone defends himself, and Poppea realizes that Agrippina has deceived her. Abandoning pretense, she tells Ottone to visit her later and begins to plot revenge on Agrippina. Lesbo arrives to tell her that Claudio is impatient to arrange another rendezvous. Poppea sees an opportunity and agrees. When Nerone enters, she lures him into an assignation at her apartment that night. Agrippina, meanwhile, is full of fear that Pallante and Narciso will betray her to Claudio, that Poppea will see through her lies, and that Ottone might still present a threat. She manages to use her charms to persuade first Pallante then Narciso back to her cause, cajoling each of them to plot the murder of the other and of Ottone. Having taken care of three enemies, she turns to Claudio, but he proves harder to persuade when she urges him to nominate Nerone as his successor to block the threat of an insurrection led by Ottone. Lesbo suddenly enters to whisper to Claudio that Poppea is expecting him. Desperate to leave, he capitulates to his wife’s nagging and agrees to name Nerone.
Ottone arrives at Poppea’s apartment, and she hides him in a closet, telling him that he will witness her revenge so long as he controls himself and stays quiet. An amorous Nerone arrives, but Poppea hides him in another closet on the pretext that she is expecting a visit from his mother. When Claudio now enters, Poppea complains that he has mistakenly ruined the wrong man; it is Nerone, not Ottone, who is Claudio’s jealous rival. Claudio is suspicious and incredulous, but Poppea reveals Nerone’s hiding place. The emperor angrily dismisses his stepson. When Poppea begs him also to leave, pretending fear of Agrippina’s revenge on behalf of her son, Claudio storms out. Poppea and Ottone are reconciled. Nerone tells his mother about Poppea’s treachery, and Agrippina pours scorn on her son for his credulity. Meanwhile, Pallante and Narciso, desperate to save themselves, reveal Agrippina’s plots to Claudio. The emperor confronts his wife with her misdeeds, but she manages to extricate herself with an elaborate and brilliant defense. She accuses Claudio in return of infidelity with Poppea. He insists that Nerone is actually Poppea’s lover and orders guards to have the others brought before him. Claudio plays with everyone’s love and ambition, throwing Poppea into Nerone’s arms and nominating Ottone once more as his successor. Ottone refuses the throne to keep Poppea. Nerone would be happy with both. Ultimately, it is Poppea who decides things, declaring her love for Ottone. Claudio blesses the union of the lovers and cedes the throne to Nerone. Agrippina is triumphant. Claudio prays for the future contentment of Rome.