The play is set in Palermo and Siena in Italy. The court of King Roberto of Sicily receives an ambassador from Roberto's ally Ferdinand, the duke of Urbino. Ferdinand has launched a military assault on the duchy of Siena, because the duchess, Aurelia, has refused his proposal of marriage. Ferdinand and his forces have taken Siena, but now face a vigorous counterattack, led by the general Gonzaga, a member of the Knights of Malta. Ferdinand has sent an appeal to his ally Roberto for help. Roberto, however, responds that the alliance between Sicily and Urbino is purely defensive in nature—the two have promised to come to each other's aid if attacked. The alliance does not cover aggressive warfare; and on that basis Roberto refuses to send any troops.