ALONG and complex Argument gives the historical background as told by Justinus (the date is approximately 89 BC, but incidents of several years are conflated), adding: ‘Upon this Historical Foundation the following Fiction is form’d, according to the Rules of Probability’ – which are considerably stretched. Tolomeo, King of Egypt, ‘depos’d by his Mother, Cleopatra, lived secretly in Cyprus, like a common Shepherd, under the Name of Osmin[o]’. His wife Seleuce, sent by Cleopatra to Syria, ‘suffered Shipwreck, and was believ’d, by every Body, to have been lost in the Sea; but, in reality, saving her self, and knowing her Husband was in Cyprus, she got over thither, dress’d likewise in a Shepherdess's Habit, under the fictitious Name of Delia, in order to find him out’. (She has had no success when the opera begins.) Cleopatra then sent her younger son Alessandro to Cyprus ‘with a powerful Army, in order to get Ptolomy into his Hands, altho’ it was really his entire Design to save his Brother, and restore him the Crown’. Meanwhile Meraspe, the King of Cyprus, ‘who, together with his Sister Elisa, resided in a delightful Village, situated in a Maritime Country of that Island … was in Love with the Shepherdess Delia, whose real Name was Seleuca; and just so was his Sister Elisa in love with Ptolomy, the reputed Shepherd Osmin; and this gives Birth to the several Incidents in this Drama’.

Act I opens in a Country by the Sea-side, with a misplaced stage direction (Alexander is helped out of the Sea by Ptolomy, and leans, fainting on a Rock). Tolomeo rails against the sea for depriving him of his wife, and against his mother for doing the same and in addition deposing him in favour of his younger brother. In despair he goes to throw himself into the Sea. A voice from within calls for assistance. Tolomeo sees a man swimming from a capsized landing-craft, helps him out of the Sea and supports him on a rock. He is astonished to recognise Alessandro, the brother whom he believes betrayed him, but decides to spare his life: ‘’Twill be a Pleasure to me to tax him doubly with Ingratitude.’