A King and No King is a Jacobean era stage play, a tragicomedy written by Francis Beaumont and John Fletcher and first published in 1619. It has traditionally been among the most highly praised and popular works in the canon of Fletcher and his collaborators.

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The tragicomedy tells that 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.

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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.

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Arbaces' dilemma is resolved when it is revealed that the situation is a complex hoax, staged by Arane and Gobrius to give an heir to the childless old king who was Arbaces' predecessor. Arane's plots against her supposed son were intended to restore the rightful succession. Arbaces is in fact Gobrius's son, and so Panthea is not actually his sister. Gobrius had plotted that his son would become the legitimate king, by marriage with Panthea; Arbaces does marry the princess, but steps down from the kingship.