A bitter father-son rivalry ignites when Don Carlos finds that his intended bride, Elizabeth, has been promised to his father, King Philip II. Will the pair ever find love under the malign eye of the Spanish Inquisition? Verdi’s swirling music with its dark undercurrents makes this a powerful evening.
Don Carlo, the heir to the throne of Spain, is in love with his young stepmother, Elisabeth of Valois. His friend Rodrigo, the Marquis of Posa, attempts to distract him and begs him to take steps on behalf of the oppressed Low Countries. Carlos asks Elisabeth to intervene with the King, Philip II. Carlos then gives his emotions free rein, at which Elisabeth reproves him, even though she loves him also. Carlos flees. The King arrives and is furious that he finds the Queen alone. Rodrigo pleads for the Low Countries with the King, but Philip tells him of his worries about Elisabeth and Carlos.
It is evening. Carlos has received an anonymous letter and is waiting for the Queen in the garden. The letter’s writer was the princess Eboli, who is also in love with him. Carlos reveals that he loves Elisabeth and rejects Eboli, who swears vengeance. Rodrigo asks Carlos to entrust any incriminating documents concerning the Low Countries that he might possess to him. An auto-da-fé is about to begin; deputies from the Low Countries approach and beg the King for aid. He sends them away, at which Carlos draws his sword and demands to be sent to the Low Countries. Rodrigo disarms him. Philip creates Rodrigo a duke and has Carlo arrested.
Philip obtains help from the Grand Inquisitor in setting up a trial for his son. Princess Eboli has passed Elisabeth’s jewel box on to the King; a portrait of Carlos is inside. The King confronts the Queen with it, at which she faints. Eboli then confesses everything, including the fact that she has been the King’s mistress. Elisabeth bans her from the Court. The incriminating documents have been found in Rodrigo's possessions. He goes to see Carlos in prison, knowing that his own last hour is upon him. A gun fires and Rodrigo falls; mortally wounded, he dies in Carlos' arms. Philip enters and goes to give Carlos back his sword, but Carlos rejects him, having realised that Rodrigo has died to save him. The populace storm the prison and Carlos flees.
Elisabeth and Carlos bid each other farewell in the gardens of the monastery of San Yuste. They are taken by surprise by the King and the Grand Inquisitor, but a mysterious monk appears (the ghost of Carlos V?) before the guards can take Carlos prisoner and drags him away into the monastery.