A marriage between the French princess, Elisabeth de Valois, and the Spanish Infante, Don Carlos, is intended to seal a lasting peace between these two world powers. The couple meet for the first time in the forest of Fontainebleau and begin to fall in love. However, for reasons of state it is decided that Elisabeth will not marry Carlos, but his father, King Philip II. She becomes Carlos’s stepmother.
Monks carry the body of Charles V. to his grave. Philip II’s father had handed the business of government over to his son and spent his final years living as a monk in a monastery.
His grandson Don Carlos cannot bear that Elisabeth has married his father and he is no longer allowed to love her. Heaven alone, it seems, can release him from this nightmare.
His friend, Rodrigo, Marquis of Posa, who has just returned from a journey through the Dutch provinces, pulls him out of his depression. He tries to fill Carlos with enthusiasm to fight together for the liberation Flanders. The Infante should petition his father to transfer government of the provinces to him. Carlos, who simply wants to escape from Spain, agrees. He and Rodrigo swear to each other that they will be friends until death.
Princess Eboli enjoys her free time in the Queen’s chambers together with the ladies in waiting, singing an oriental veil song which is actually forbidden at court. When Elisabeth de Valois appears, the relaxed mood changes. Suddenly Marquis Posa also enters and secretly hands the Queen a letter, in which Carlos asks to speak to his stepmother. Rod-rigo urges her to grant the unhappy Carlos an audience. Eboli, who is secretly in love with Carlos, interprets this hopefully as a sign that he might return her love.
Elisabeth agrees and receives Carlos alone. He is unable to conceal his feelings for her and confesses his love once again. When she makes it clear to him that the situation is hopeless, he runs away.
Philip II is surprised to find his wife alone and banishes the French lady in waiting who strict court protocol demands should be in the Queen’s presence at all times.
After everyone else has left the room, Philip II asks Marquis Posa to stay. Now that he has to chance to speak to his ruler openly, Posa tells him of the cruelty of the Spanish regime in the Netherlands. Despite this strong criticism, the King shows considerable sympathy for the young idealist and makes him his closest adviser. He confides in him that he suspects his wife of having an affair with his son. Philip also warns his new confi-dante about the Grand Inquisitor.
Princess Eboli has written Carlos an anonymous letter summoning him to a midnight ren-dezvous. Don Carlos believes Elisabeth has written the letter and greets the veiled woman passionately. He does not recognize Eboli until it is too late. She realises the true nature of his feelings and swears that she will avenge herself on both Carlos and the Queen. Even Rodrigo is unable to dissuade her. Looking for a way out of the problem, Posa asks Carlos for all the letters and papers which could incriminate him as a political agitator or as the Queen’s presumed lover.
Prisoners are being publicly executed by the Inquisition in an auto da fé. Carlos suddenly interrupts the proceedings together with ambassadors from Flanders and rebuffs the King in front of the assembled court and church representatives. Those watching think he has gone mad. Don Carlos urges his father to entrust the provinces to him and ultimately threatens him in public with a weapon. Rodrigo disarms him and brings the protest to an end. The Flemish prisoners are burnt to death.
Eboli hands Philip II his wife’s casket of jewels. Inside it he finds his son’s portrait. The King is pained by his failure as a husband and father, but also by his impotence in the face of the Catholic church.
Nevertheless he asks the Grand Inquisitor to advise him whether a father is permitted to kill his son. The Inquisitor offers him absolution for this deed, but demands in return that Marquis Posa, the real protagonist of the rebellion, be handed over.
Elisabeth has noticed that her casket has been stolen and demands justice from the King. He accuses her of adultery and when she defends herself, he loses control. He hits his wife who loses consciousness and falls to the floor. He then calls Posa and Eboli for help. Rodrigo decides to divert all the suspicions against Carlos onto himself in order to save his friend. Eboli confesses to Elisabeth that she gave the casket to the King out of jealousy. The Queen banishes her. Eboli attempts to use the few hours she has left before she must leave to save Carlos’s life.
Rodrigo visits Carlos in prison, in order to say farewell. He is now on the run from the Inquisition, which has charged him with treason. The assassins’ bullets find Rodrigo in Carlos’s arms. Before he dies, Rodrigo just has time to tell his friend that Elisabeth will meet him the next day for a final conversation. Philip II tries to make peace with his son, but Carlos explains how the Marquis sacrificed himself for him. Meanwhile Eboli has incited a crowd to storm the prison. They demand to see the Infante. The Grand Inquisitor comes to the King’s aid and forces the crowd to give way. Carlos, however, is able to escape.
Elisabeth waits for Carlos at Charles V’s grave. When he arrives, she implores him to fulfil Rodrigo’s last wish and liberate Flanders. It will be impossible for them to live together in this world, so they bid each other farewell. However, the King and the Grand Inquisitor are both watching them. Philip II kills his son. The voice of Charles V is heard, promising redemption in death from all earthly suffering.