Don Carlo and Elisabeth de Valois, princess of France, are betrothed; however, for reasons of state, the princess is obliged to marry Carlo’s father, King Philip II of Spain. Carlo confides his love to Rodrigo, Marquis of Posa, who advises him to forget Elisabeth and ask for the governorship of Flanders, where he may ameliorate the hardships of Spain’s reign. Philip refuses his son’s request in great anger.

The Princess Eboli, Elisabeth’s lady-in-waiting, whose passionate love for Carlo is unreturned, informs the King of Carlo’s love for Elisabeth. Philip orders his rebellious son thrown into prison, and questions the Grand Inquisitor on the propriety of asking for a death sentence. The Inquisitor demands instead that the king denounce Rodrigo, a greater menace because of his liberalism. Philip refuses. Thoroughly embittered, the King rails at Elisabeth for her supposed unfaithfulness.

Eboli repents her rash confession and determines to rescue Carlo from prison. She accomplishes this mission in the confusion that ensues after the treacherous shooting of Rodrigo, who has been marked for death as he visits his friend. Carlo and Elisabeth meet in the monastery cloister, where a monk at the tomb of Charles V steps forward to rescue Carlo from the pursuing King and Inquisitor.