On his return Arbaces finds that he now has a powerful sexual attraction to his beautiful sister, the princess Panthea, whom he hasn't seen since childhood. Much of the play depicts his increasingly desperate struggle against his incestuous passion. Arbaces blames the protector Gobrius for his predicament; the minister had written Arbaces many letters during the king's years abroad, praising Panthea's beauty and her love for him. Panthea is also attracted to Arbaces, but her virtue restrains them both. The king becomes so desperate that he decides to murder Gobrius, rape Panthea, and then commit suicide. Meanwhile, Tigranes too falls in love with Panthea, even though this means he breaks his faith with Spaconia. Tigranes exercises the self-discipline and rationality that Arbaces struggles to achieve, and rededicates himself to Spaconia.