THE scene is Alexandria in the year 80 bc and the background of the plot a dynastic struggle between Rome, governed by the dictator Sulla and represented by his ambassador Fabio, and Mithridates King of Pontus, Rome's long-standing enemy. The young Berenice has recently been proclaimed Queen of Egypt on the death of her father Ptolemy VIII without male issue. Sulla and Mithridates (neither of whom appears in the opera), evidently not trusting a woman, each want her to marry a politically friendly nominee. Sulla's candidate is Alessandro, a tame Egyptian (son of another Ptolemy) who has been brought up in Rome; that of Mithridates is Demetrio, described simply as ‘a Prince of Royal Blood’. Berenice is in love with Demetrio, and has to choose between marrying for love or for political expediency, since her throne is threatened by the dominant power of Rome. The brief Argument concludes: ‘To this historical Fact [Alexander's embassy to Egypt as told in Book I of Appian's Roman History] are added other similar Incidents, as will be seen in the Drama.’

Act I. A Hall with a Throne, and Satrapes, who sit. Berenice enters with Aristobulus (her Captain and Confidant) attended by Guards and Pages. All the Satrapes rise from their Seats. Struggling to control her passions, she tells Aristobolo to admit the Roman ambassador. She ascends the Throne, and the Satrapes sit. Fabio, entering with Alessandro and Roman Attendants, brings peaceful greetings from Sulla and the senate, takes Alexander by the Hand and presents him, and urges Berenice to marry him, a friend of Rome. She refuses to submit to any domination but that of her own heart, and goes out followed by the Satrapes, and by her Attendants. Alessandro has been struck by her beauty and proud demeanour, but Fabio brusquely advises him to transfer his affections to Berenice's sister Selene, who also loves Demetrio. Alessandro should have ‘more manly Sense’ and ‘love where Profit, not where Fancy calls’, taking his cue from the bee that flits from flower to flower. Alessandro says he can love only Berenice. In a Garden of the Apartments of Selene Selene tells Demetrio that Berenice has ordered them never to meet again and urges him to depart.