Act I

The opera is set in the country villa of the Roman Emperor Ottone, with its delightful gardens containing avenues of cedars, ponds and fountains. Ottone is besotted with the beautiful Cleonilla, who is revealed as the opera opens 'gathering flowers to adorn her bosom'. She reveals that, despite being loved by the Emperor, she finds it impossible to resist the attractions of any handsome young man. One such old flame was Caio Silio, but he has recently been replaced in her affections by her new page, Ostilio. Cleonilla protests to Caio that she still loves him, although an aside reveals that she now finds Ostilio even more attractive! Ottone arrives, looking forward forgetting weighty affairs of state in his beautiful surroundings, but Cleonilla teases him, claiming that he cannot truly love her as he spends so little time with her. Ottone asks Caio to help him cure her jealousy, while Caio marvels at the Emperors credulity. Tullia now enters. Formerly betrothed to Caio, she has followed him disguised as a man and is, in fact, none other than Ostilio. 'Ostilio' asks Caio if he still remembers his betrayal of the unfortunate Tullia. Caio, while noting the striking resemblance of the page to Tullia, does not guess me truth; he declares that his new love for Cleonilla has driven Tullia from his mind, Questioning in his ensuing aria the merits of constancy, since 'love becomes a burden without variety', 'Ostillio's' thoughts turn to revenge.

The scene changes to a bathing-pavilion, where Cleonilla is emerging from her bath. She is still teasing Ottone, but they are interrupted by Decio, Ottone's faithful advisor, who tells the Emperor that Rome is unhappy at his absence. Ottone is unconcerned, but, when he has left, Cleonilla quizzes Decio on what they are saying about her in Rome. Decio has no time for her lasciviousness, and his aria (whose text replaces that found in the 1713 printed libretto) tells her that she is deceiving herself if she believes that the love of a king can make up for the lack of true honour. As Decio departs, 'Ostilio' arrives, and Cleonilla immediately declares her love for him. 'Ostilio' seizes on this as a means for revenge against Caio and encourages Cleonilla to swear an oath of love for him and abhorrence of Caio. Caio, who has overheard all this, is horrified, and he resolves to reveal 'Ostilius' treachery to the Emperor, ending the act with a fiery aria describing his jealousy and bitter grief.

Act II

In a delightful sunken garden, Decio warns Ottone that Cleonilla will be his downfall, for Rome disapproves of her numerous well known affairs. Ottone is astounded, and, in a typical eighteenth-century simile aria, likens his turbulent feelings to violent waves on a stormy sea. Decio reveals that he deliberately refrained from telling the Emperor that Caio is his rival, but he will not explain to Caio what has upset Ottone so much. Caio, apparently left alone, muses on his misery, but he is overheard by the hidden Tullia who responds to him in the manner of an echo. The echo, claiming to be the voice of an unhappy spirit, torments Caio, whose distraught feelings are portrayed in a short section or accompanied recitative and the ensuing echo aria. 'Ostilio' now reveals himself and sings of the conflict in his heart between the 'two tyrants'; indignation and love.

The scene changes to a rustic lodge, where Cleonilla is admiring herself in a mirror. Caio enters, but his protestations of love are casually dismissed. He gives her a letter declaring his feelings, but, as she is about to read it, Ottone arrives and snatches it from her. He reads that Caio is his rival, but Cleonilla tells him that Caio has simply given her the letter to pass on to its true addressee, Tullia, who has been unfaithful to him. The gullible Ottone believes her, and she adds to her deception by writing a second letter - her own appeal to Tullia - which she asks Ottone to deliver. Decio arrives with further news of plotting in Rome, but Ottone still will not hear a word against Cleonilla, and he sends for Caio. He chides the guilty Caio, who initially believes that the game is up but then realises, to his great relief, that Ottone is annoyed not because he has discovered the affair with Cleonilla but simply because Caio sought Cleonilla's help instead of turning to his Emperor. Left alone, Caio is impressed by Cleonilla's cunning, while in the act's final scene the distraught 'Ostilio' begs 'Love to come to his aid'.

Act III

On a secluded, leafy path, Decio once again tries to persuade Ottone of the danger he faces from Rome, but the Emperor declares in his aria that he cares nothing for throne or empire, so long as he has happiness in love. Decio prophesies Ottones imminent downfall, for love in a ruler is a sign of weakness, but he is interrupted by the arrival of Cleonilla and Caio. She is still ignoring the latter's advances, and when 'Ostilio' appears she addresses alternate words of love to him and rejection to Caio. Caio pretends to take her advice and leave, but in fact he conceals himself. Cleonilla continues to protest her love for 'Ostilio', who encourages her in his aria while revealing in an aside that she is making a mistake. The sight of the pair embracing is too much for Caio, and he rushes at 'Ostilio' with a dagger. Cleonilla's cries alert Ottone and Decio, whose entry provokes an explanation from Caio. He describes the scene that he has just witnessed - Cleonilla and 'Ostilio' embracing - and the shocked Emperor commands him to complete his task and kill the traitor. 'Ostilio' offers to explain, however, and removes his disguise, reveals himself as the wronged Tullia. Now in her true guise, she professes Cleonilla's innocence and accuses Caio of being the real traitor. All are astonished, although Ottone regains his composure with remarkable rapidity, expressing his desire to see Caio and Tullia married and asking Cleonilla for forgiveness. He opera ends with an ensemble of general rejoicing.