When her husband, the Roman emperor Claudius (Claudio), is apparently drowned at sea, Agrippina plots for her son Nero (Nerone) to be his successor.
In fact Claudius has been saved by Otho (Ottone) and the immminent coronation of Nero is abandoned.
Otho arrives and tells Agrippina that Claudius, in gratitude, has appointed him his successor. He also tells her that he is in love with Poppea.
Agrippina, aware that Claudius also loves Poppea, tells Poppea that Otho has agreed to give her to Claudius in return for the crown. She suggests to Poppea that by telling Claudius that Otho has refused Poppea access to him, the emperor will dismiss Otho from the throne.
Otho claims his reward from Claudius who denounces him as a traitor. He is then vilified by Agrippina, Poppea and Nero.
But Poppea begins to doubt his guilt, and eventually Otho convinces her of his innocence.
Agrippina then tells Claudius that Otho is plotting against him and persuades him to appoint Nero emperor.
Poppea explains to Claudius that whereas she once thought Otho had betrayed him, it was in fact Nero, whom she then reveals hiding behind a curtain in her room. Claudius dismisses his stepson, who informs his mother of Poppea's treachery.
Agrippina confronts Claudius, berates him for succumbing to Poppea's influence, and claims that Otho loves Poppea, thereby forcing Claudius to summon all three.
He orders Nero to marry Poppea and leaves the succession with Otho, who requests that he might forgo the crown for Poppea's hand in marriage. Claudius agrees, and blesses Poppea and Otho's marriage.