Scene I. The Cloisters of the Monastery of San Juste.
A friar is praying at the tomb of Carlos V, Don Carlosґ grandfather. Carlos is in despair because his father has married Elisabeth, to whom he was once betrothed. The Friarґs voice reminds him of his dead grandfather. Carlosґ great friend Rodrigo, the Marquis of Posa, enters. Carlos confides to Posa that he and Elisabeth are still in love. Rodrigo advises him to forget Elisabeth and take up the cause of the Netherlands, oppressed under his fatherґs rule. Carlos agrees and, after they have watched Philip II and Elisabeth pray at the tomb, they swear undying friendship.
Scene II. A garden outside the cloisters.
The Queenґs ladies are assembled for her return and Princess Eboli entertains them with The Song of the Veil. The Queen returns with Rodrigo. Under cover, he manages to pass her a private note form Carlos, who wishes to see her. She hesitates but, encouraged by Rodrigo, consents to the meeting. She sends her ladies away and Carlos enters. She agrees to persuade Philip to send Carlos to the Netherlands as governor, but cites her duty when accused of indifference to his love. He becomes impassioned and, when she resists him, rushes off. Philip enters, furious to find Elisabeth unattended. He exiles the Countess dґAremberg for negligence and the court departs. Philip draws Rodrigo back and asks him for frank advice. Rodrigo speaks for the Netherlands, but Philip warns him of the Grand Inquisitor. Philip confides his suspicions of Carlos and his wife, and asks Rodrigo to watch them. Encouraged that the Kingґs confidence may bring influence, Rodrigo agrees.
Scene I. The Queenґs Garden.
Carlos re-reads an anonymous letter – perhaps from Elisabeth – asking him for an assignation. A veiled woman appears and he embraces her, declaring his love. But when she removes her veil he finds not Elisabeth, but Eboli. His confusion persuades Eboli that it is Elisabeth he loves. Rodrigo arrives and tries to divert her. But Eboli loves Carlos herself and decides to reveal the truth to the King. Carlos threatens her and they quarrel furiously. She leaves, bent on vengeance. Rodrigo wants Carlos to give him any incriminating documents he may have. Carlos is doubtful because of Rodrigoґs newfound favour with the King, but warily gives him a cache of letters.
Scene II. The Square outside the Cathedral of Our Lady of Atocha.
A crowd has assembled to watch an auto-da-fe. Elisabeth and Rodrigo arrive in procession and Philip makes a grand appearance from inside the cathedral. Carlos leads a deputation requesting leniency in the Netherlands, and Philip orders them to be removed. The royal party joins in the plea, but the monks oppose it. Philip rejects Carlos as governor of the Netherlands and Carlos draws his sword. The King defends himself and Rodrigo, to protect his friend, demands that he surrender his sword. Astonished, Carlos does so, and is led off under guard. Flames rise from the stake and a heavenly voice welcomes the souls of the burning "heretics".
Scene I. The Kingґs study.
Philip reflects on his loveless marriage and feels that he will die alone. The Grand Inquisitor arrives and Philip asks him how to deal with his sonґs rebellion. The Inquisitor insists on Carlosґ death, and then demands that the Inquisition interrogate Rodrigo. When he has gone, Elisabeth rushes in, believing her jewel casket stolen, but Philip has it. He forces it open to find Carlosґ portrait and accuses Elisabeth of adultery. She swoons. Philip calls out and Eboli enters with Rodrigo. As the Queen recovers, they both voice their emotional conflict. Eboli is then left with the Queen and confesses that she gave Philip the casket and that she has also been his mistress. Stunned, Elisabeth orders Eboli to choose between exile and the convent. The Queen leaves and Eboli bitterly regrets the curse of beauty that has been her downfall. She chooses the convent, but first vows to save Carlos.
Scene II. A prison cell.
Carlos is visited by Rodrigo, who has come to sacrifice his own life for his friendґs. There is little time because he has arranged for the letters to be found on his own person. An Inquisition officer steals into the cell and shoots Rodrigo. Before he dies, Rodrigo tells Carlos that the Queen will be waiting for him at San Juste the next day. The King enters, but when he attempts to embrace his son, Carlos pushes him away. A crowd storms the prison and demands Carlosґ release. In the confusion, Eboli begs Carlos to leave and the Grand Inquisitor arrives to quell the mob.
The Cloisters of the Monastery of San Juste.
Elisabeth enters to pray at Carlos Vґs tomb. Don Carlos arrives, telling her that the Netherlands, not their past love, now call him. They bid farewell, but Philip enters with the Grand Inquisitor and his officers, and orders both their arrests. Carlos retreats toward his grandfatherґs tomb and the friar, whom he had earlier heard prying, suddenly appears dressed in the royal robes and regalia of Carlos V. He draws Don Carlos into the cloister as the others watch in awe.