Act I
When her husband, the Roman emperor Claudio, has apparently drowned at sea, Agrippina plots for her son from a previous marriage, Nerone, to be Claudio’s successor. In fact, the emperor has been saved from death by the commander of the army, Ottone, and the imminent coronation of Nerone is abandoned. Ottone tells Agrippina that Claudio, in gratitude, has appointed him his successor. He also tells her that he, Ottone, has fallen in love with Poppea, and that he desires her more than he desires the throne. Agrippina, aware that Claudio also loves Poppea, sees a new opportunity of furthering her ambitions for Nerone. She goes to Poppea and tells her, falsely, that Ottone has struck a bargain with Claudio whereby he, Ottone , gains the throne but gives Poppea to Claudio. Agrippina advises Poppea to turn the tables on Ottone by telling the emperor that Ottone has ordered her to refuse Claudio's attentions. This, Agrippina believes, will make Claudio revoke his promise to Ottone of the throne.
Act II
Ottone is ready for his coronation but Claudio instead denounces him as a traitor. Devastated and bewildered, Ottone appeals to Agrippina, Poppea and Nerone for support, but they all reject him. Poppea is touched by her former beloved’s grief and begins to doubt his guilt. Eventually Ottone convinces her of his innocence and Poppea swears revenge, but she is distracted when Nerone comes forward and declares his love for her. Meanwhile, Agrippina convinces Claudio that Ottone is still plotting to take the throne and advises him to abdicate in favour of Nerone. Claudio agrees, believing that this will enable him to win Poppea.
Act III
While Ottone and Nerone hide in her bedroom, Poppea explains to Claudio that he had earlier misunderstood her: it was not Ottone but Nerone who had ordered her to reject his attentions. Claudio angrily dismisses his stepson. Nerone informs his mother of Poppea's treachery and of his decision to renounce love for political ambition. Agrippina confronts Claudio, berates him for succumbing to Poppea's influence, and claims that Ottone loves Poppea, thereby forcing Claudio to summon all three of them. He announces that Nerone and Poppea will marry, and that Ottone shall have the throne. No one is satisfied with this arrangement, as their desires have all changed, so Claudio, in a spirit of reconciliation, reverses his judgement, giving Poppea to Ottone and the throne to Nerone.