Arbaces, the valiant young king of Iberia, ends a long war against Armenia by defeating in single combat Tigranes, the king of that country. Arbaces, although a hero in war, is also an intensely passionate man; honest and outspoken Mardonius comments that he is capable of the wildest extremities of emotion and that he can move through the entire emotional range with the greatest speed. Inflamed by his victory, Arbaces illustrates the qualities Mardonius ascribes to him. In a series of blustering speeches he shows himself to be inordinately proud. When Mardonius takes him to task for boasting, he becomes, after a few gusts of ranting, temporarily contrite and amiable, and he resolves to give his beautiful, virtuous sister Panthea, whom he did not see since her childhood, in marriage to the defeated but valorous Tigranes. Tigranes protests because he already plighted his troth to Spaconia, a lady of his own land.

Messages, arriving from Gobrias, in whose care the government of Iberia was left, tell that a slave sent by Arane to poison Arbaces was taken and executed. Instead of flying into a rage, Arbaces, in a burst of magnanimity and pity, forgives the queen mother’s unnatural act. Thus he swings from the objectionable boastfulness of moments before to the opposite emotional pole.

Meanwhile, Tigranes, who is to accompany Arbaces home as a prisoner, arranges with Bessus, a fatuous and cowardly captain in the Iberian army, for him to convey Spaconia to Iberia and secure for her a place as one of Panthea’s ladies-in-waiting. There, according to Tigranes’ plan, it is to be Spaconia’s task to set the princess’s heart against a match with him.

In Iberia, where Arane was put under guard for her attempt on Arbaces’ life, Panthea is deeply torn between her love for her mother on the one hand and her loyalty and devotion to the king, her brother, on the other. Although the reason for Arane’s crime is unexplained, her conversation with Gobrias reveals that there are secrets between them having an important bearing on her relationship with Arbaces. Bessus, accompanied by Spaconia, arrives with messages from the king, including a pardon for Arane. Importuned by the courtiers, the braggart gives an amusing account of the duel between Arbaces and Tigranes, contriving to make himself the central figure. Panthea, interrupting Bessus’s tale frequently, reveals agonized concern for her brother’s safety. Even though she did not yet see him, she nevertheless feels a powerful attraction to him. Spaconia then reveals to Panthea her reason for coming to Iberia, and the virtuous princess vows to reject the proposed match with Tigranes.

After a triumphal passage through the city, Arbaces and his company arrive at the court. When Panthea presents herself to her brother, Arbaces, overwhelmed by her beauty, falls hopelessly in love with her at first sight. Frantically he tries to convince himself that she is not really his sister but a lady of the court; however, he is unable to escape the guilty feeling that he is the victim of an incestuous love.

At last, succumbing to his passion, he kisses her; then, overcome with guilt and shame, he violently orders the weeping Panthea imprisoned. As time passes, however, his love for Panthea increases, and at last he begs Mardonius to act as his bawd. When Mardonius indignantly rejects Arbaces’ plea, the king turns to Bessus, whom he finds more willing to undertake such a task. Revolted by Bessus’s ready acquiescence, and probably also by the image of himself that he saw in the minion, Arbaces swears to keep his sin within his own breast...