THE background to the plot is complex and not made much clearer by the Argument in the printed libretto. Adelaide, daughter of the Duke of Burgundy and King of Italy, succeeded her father as Queen Regnant and married Lotario, son of the Count of Arles. Despite their paternal rule the people rebelled, led by Berengario Duke of Spoleto. Lotario voluntarily shared his kingdom, reigning at Pavia, Berengario in Milan. Berengario, fired by ambition to possess the whole kingdom, had Lotario poisoned and schemed for his son to marry the widowed Adelaide. She refused the match and was besieged by Berengario in Pavia. Her uncle the Marquis of Tuscany sent for help from Ottone King of Germany (the future Emperor Otto I). Confusingly Handel changed the name of Ottone to Lotario halfway through the composition (on fol. 66 of the autograph), presumably because he had already composed an Ottone. The period is the middle of the tenth century. Matilde, Berengario's wife, and her son Idelberto are the same persons as Gismonda and Adelberto in Ottone; the Ottone Matilde has no counterpart here.

Act I. A Garden outside the city of Pavia. Berengario with Attendants meditates on a ruler's hopes and fears. Idelberto, though he loves Adelaide, feels no hope of winning her. Berengario brusquely orders him to storm the city. Clodomiro, Berengario's general, reports that Lotario has crossed the Alps with a large army to support Adelaide. Berengario orders the assault to begin before he can arrive. Matilde says Pavia is already theirs; she has bribed enough of its defenders. Idelberto, shocked at such underhand proceedings, kneels before his father, who sends a herald to Adelaide: he will return her kingdom if she consents to marry Idelberto, otherwise let her fear his vengeance. Idelberto appeals to his mother, who tells him to go and make love to Adelaide, explaining the alternatives. He would rather die than force a hated marriage on her. In an Audience Chamber with a Throne in Pavia Adelaide with Attendants laments the precarious state of her throne and the treacherous murder of her husband. Turning to the Guards, she orders the admission first of the warrior who asked for a private audience, and then of Berengario's envoy. One of the Guard withdraws. The Guards introduce Lotario and retire.