Setting: Rome, circa 50 A.D. When the Emperor Claudius is reported to have drowned, his wife Agrippina plots to have Nero, her son by a previous marriage, succeed him. Agrippina’s plan appears to work until the news arrives that Claudius has been rescued. Otho, the soldier who heroically saved the Emperor, returns to the Capitol and announces that Claudius has promised him the throne. Agrippina devises a new plot, exploiting Otho’s overwhelming love for Poppea. Knowing that Claudius also desires Poppea, Agrippina tells the young beauty that Otho has ceded her to the Emperor in order to obtain the throne. She suggests that Poppea take revenge by convincing Claudius to revoke Otho’s regal reward. Their plan is successful; in his fury, the Emperor deems Otho a traitor and swears to rescind his promise.

Otho reminds Claudius of his promise, but the angry Claudius turns him away. Otho finds no compassion from Agrippina, Nero, or Poppea, and sinks into despair. Seeing Otho’s grief, Poppea begins to doubt his guilt. After discovering the truth, Poppea sees through Agrippina’s manipulations and swears vengeance. Meanwhile, Agrippina is tormented by the failure of her schemes. She tells Claudius that Otho wants revenge for his loss of the throne and urges him to declare Nero his successor. Claudius, impatient to go to Poppea, agrees. Agrippina anticipates her son’s reign.

Poppea develops a plan to right the injustice done to Otho. She hides Otho in her bedroom and receives Nero – who also desires her – as well as Claudius, ensuring that each rival for her affection overhears other suitors. After dismissing Nero, Claudius is sent away by Poppea; she and Otho swear their eternal fidelity. Nero tells Agrippina what has transpired and asks for protection from Claudius. Agrippina once again urges Claudius to crown Nero, but he accuses Agrippina of trying to usurp his power. She in turn accuses Claudius of betrayal and asks him to abandon his love for Poppea, telling him that she is Otho’s lover. An argument ensues and, in the turmoil, the emperor suddenly announces that he shall crown Otho as his successor, with Nero marrying Poppea. However, as this pronouncement satisfies no one, Claudius finally gives the throne to Nero and unites Poppea and Otho, to general rejoicing.