Agrippina has great news for her son Nerone: Emperor Claudio, Agrippina’s husband and Nerone’s step-father, is reported to have drowned in a storm on his journey back from Britain. The time has therefore come for Nerone to claim the throne. Agrippina instructs her son to walk among the people and to show humility, modesty and generosity. She then summons the freedmen Narciso and Pallante, one-by-one. Both were influential advisers at Claudio’s court and are in love with Agrippina. She promises both that she will reciprocate their love and even allow them to reign together with her if they will support Nerone. By means of Nerone's coronation Agrippina obviously intends to secure her own influence.
Nerone distributes alms at the Capitol to shouts of acclamation from Narciso and Pallante. Agrippina joins in. However, Claudio’s servant Lesbo then announces that the Emperor did not drown, but was snatched from the waters by the gallant Ottone.
Ottone reports that Claudio promised him the throne in gratitude for his valiant deed. Agrippina is the only one to maintain her composure and congratulates the hero, who proceeds to request a private conversation. He professes his love for Poppea and that he would renounce the throne in order to be with her. Agrippina knows that Claudio, too is secretly in love with Poppea. She leads Ottone to believe that she will lend him her support.
Poppea is flattered by the declarations of love she has received from Ottone, Claudio and Nerone. Nobody knows her true plans and desires, though she in fact loves Ottone.
Agrippina, with seemingly the best of intentions, warns Poppea about Ottone: He has promised Claudio that he can keep her as his lover, in exchange for the crown and sceptre. Poppea is shocked, taking Agrippina at her word. Agrippina advises her to speak ill of Ottone to Claudio, as a means of avenging his deceit.
Poppea appears sorrowful around Claudio. She claims that Ottone, assured of becoming Emperor, has forbidden her from loving Claudio. Furious, Claudio vows never to allow Ottone to become Emperor.
Narciso and Pallante feel betrayed by Agrippina, but also fear the rage of Claudio, due to their support for Agrippina’s plan to make Nerone Emperor. They pledge to help each other. Together, they greet Ottone as the future Emperor. Ottone himself is only interested in Poppea, though she responds to him coldly. Agrippina’s suggestive whisperings cause Poppea’s anger towards Ottone to burn even more fiercely.
Following Claudio’s triumphal entrance, the Emperor, deeply moved, receives his subjects’ adulation. As Ottone steps forward and humbly requests the reward for his act of heroism, Claudio erupts in a fit of rage, accusing Ottone of being a traitor deserving only death. Ottone, unable to comprehend what has befallen him, asks Agrippina, Poppea and Nerone one-by-one for help, then Pallante, Narciso and Lesbo for sympathy, but all walk away.
Poppea takes pity on Ottone. She wishes to know how he will defend himself. As Ottone, vacant and downcast, passes by she confronts him, as though talking in her sleep, accusing him of conceding her to Claudio in return for the throne. Ottone protests his innocence to Poppea, now seemingly awake. Poppea hesitates; after all, she did receive the information from Agrippina. Ottone is outraged at Agrippina’s unscrupulousness and begs Poppea to believe him. She tells him to visit her at night, when a decision on his guilt will be reached.
Poppea begins to see through Agrippina’s intrigues and vows to avenge this deception. Via Lesbo, she summons Claudio to her chambers for the night, but then invites Nerone to visit at the same time.
Agrippina is tormented by gloomy thoughts. She regrets to have admitted Narciso and Pallante too much into her confidence and sees her plans jeopardized by Ottones and Poppeas virtue and courage. After a troubled moment, she reacts with determination: First, she charges Pallante with killing Narciso and Ottone, before instructing Narciso to murder Pallante and Ottone. Both appear to agree to her request. Agrippina now speaks to Claudio’s conscience. She suggests that his life will be under threat, given that Ottone is lamenting the injustice done to him, thereby stirring up others against Claudio. Only the immediate naming of a new Emperor can put an end to Ottone’s machinations and loyal Nerone would be the most suitable choice. Claudio hesitates at first, but urged to leave by Lesbo, promises to hand over the throne to Nerone before hastily departing. Satisfied, Agrippina reaffirms her pledge to risk everything for the sake of her mission.
Poppea and Ottone are once again happily united. Ottone was able to convince his beloved of his innocence. Poppea announces her plan to take revenge on Agrippina for her scheming.
After Ottone has hidden, Nerone enters. Poppea declares her love for him, but urges him to also hide himself, under the pretext of Agrippina’s imminent arrival. When Claudio now appears, Poppea clarifies a supposed misunderstanding, namely that it is not Ottone, but Nerone who has been bothering her. In order to prove Nerone’s guilt she leads Claudio to his hiding place. Claudio is enraged and chases Nerone away. Once Poppea has persuaded Claudio to leave, Poppea and Ottone declare their love for one another.
Nerone informs Agrippina of what has happened. She accuses him of jeopardising their mutual endeavour for the sake of blind passion. Nerone admits his mistake, but adds that Poppea has already seen through Agrippina’s plot. Agrippina, though, reaffirms her hope and singular aim to put Nerone on the throne. Nerone’s love for Poppea grows cold as a result of his rage at her betrayal.
Narciso and Pallante inform the Emperor of Agrippina’s machinations. When she urges Claudio to fulfil his promise, he confronts her with these accusations. Agrippina calmly explains that Narciso and Pallante were witnesses to her honourable actions: She had wanted to name Nerone Emperor simply to prevent the kingdom from falling into the wrong hands. Yet, when Claudio returned, she had submitted to him gladly and immediately. Taken by surprise, the freedmen confirm Agrippina’s version of events. Agrippina now begins to accuse Poppea of treachery, but is interrupted by Poppea‘s arrival, together with Nerone and Ottone.
Claudio wishes to bring calm and peace into the general disorder. He decrees that Nerone and Poppea should marry and that Ottone should become Emperor. The concerned parties declare themselves wholly dissatisfied with this solution. In response, Claudio states that his suggestion was merely a test of their true desires and revises his decision: Nerone is to become Emperor; Ottone and Poppea shall be allowed to marry. Everyone is happy, yet none more than Agrippina: “Now that Nerone is Emperor, I will die content.”