Demetrio e Polibio, dramma serio in two acts to a libretto by Vincenzina Viganò Mombelli, was first performed at Teatro Valle di Roma, on 18th Mayo 1812. The singers in the first performances were Domenico Mombelli (Demetrio-Eumene), Lodovico Olivieri (Polibio), Ester Mombelli (Lisinga), Marianna Mombelli (Demetrio-Siveno).
A hall of audience in Polibio’s palace. Polibio reassures Siveno of all his affection for him and expresses his desire that on that very day Siveno might marry Lisinga, which fills Siveno with joy. Once Siveno has left, Demetrio comes on in his assumed character of Eumene (by which name we shall refer to him from now on) bearing rich gifts and accompanied by a train of followers, to ask, in the name of the King of Syria, that Mintèo’s son be restored to his own country since Mintèo was particularly esteemed by Demetrio. Polibio gives a decided refusal, and when Eumene threatens him replies that he has no fear of Syria’s reprisals.
The interior of a temple. In expectation of his wedding ceremonies Siveno is awaiting Lisinga, whose arrival is heralded by a festive chorus. Full of joy, Lisinga looks forward to being married. Before the altar the young couple exchange vows of eternal love. However, after the ceremony Polibio has to warn the couple that the Syrian ambassador has threatened war on Parthia unless Siveno returns home. Siveno takes his father-in-law’s part, and Lisinga says that she too, if need be, will take arms. When Lisinga has left, Siveno comforts Polibio.
A Square. Eumene tells his companions that, in view of Polibio’s refusal, he has decided to kidnap Siveno, and to this end he has bribed the guards.
The King’s apartments. It is night. Lisinga is preparing for bed, full of anxious thoughts. Eumene, stealing in, at first mistakes her for Siveno, but when he discovers his mistake he decides, in any case, to take her as a hostage. In the midst of all this confusion a fire breaks out and Siveno and Polibio are attracted by the noise. They can do nothing but look on helplessly as Eumene carries off the fainting Lisinga.
The King’s apartments. Polibio rushes about in despair, surrounded by the grandees of the realm. Siveno comes in to rouse him: he has discovered where Lisinga is held prisoner by Eumene, and calls on all present to join in the effort to free her.
Outside the city. Eumene is leading Lisinga away, escorted by his faithful followers. He has managed to reassure Lisinga that his intentions are honourable, in that he will respect her position as the beloved of Siveno, when he is surprised by the arrival of Polibio and Siveno with their men. Eumene threatens to kill Lisinga if Siveno is not given up to him, while Polibio makes the counter-threat that he would kill Siveno rather than surrender him to Eumene. Now Eumene notices a medal that Siveno wears round his neck, and recognizes him as his son. The opposing factions exchange hostages. However this temporary truce is shattered because Lisinga and Siveno cannot bear to be separated. In spite of all protests, the two couples go off in opposite directions. Left alone with Siveno, Eumene tells him that he is his father. Siveno asks his father to forgive him, but begs to be reunited with Lisinga.
Audience chamber. Lisinga, in warlike mood, decides to put on armour and prepare to get Siveno back, fully encouraged by Polibio. After she has harangued the grandees of the realm, they all go off.
Near the Syrian camp. Eumene relates that Siveno had insisted on going away to find Polibio and persuade him to allow Lisinga and himself to be reunited. Lisinga and her followers come on, intent on killing Eumene, who believes that his own son has betrayed him. However, Siveno himself now reappears and throws himself between Lisinga’s sword and his father’s chest. Eumene, deeply moved, embraces both the young people. They are all about to go and give Polibio news of the latest developments, when Polibio himself comes on. Any remaining perplexities are cleared up by Eumene, who reveals himself coram populo as the King of Syria and the father of Siveno; he then proposes an alliance to Polibio, which will be sealed by the marriage of their children. The curtain falls on the general rejoicing.