Arbaces, King of Iberia, has been abroad, fighting in the wars, for many years; he returns home in triumph, bringing with him Tigranes, the defeated king of Armenia. He intends to marry his sister Panthea to Tigranes. Meanwhile, he learns that his mother, Arane, who hates him, has plotted his assassination. The regent Gobrius has foiled the plot. Tigranes' fiancee Spaconia accompanies him into exile, hoping to avert Arbaces' plans for the marriage alliance. Tigranes promises her he will remain faithful.