As ACT ONE opens, Don Carlo, the son of Spain's King Phillip II, has returned home after a visit to France. He had briefly been engaged to a French woman, Elisabeth. The marriage was arranged to help cement a peace agreement between the two countries — but Carlo and Elisabeth actually did fall in love. Then Carlo's father Phillip decided it would be better if he married Elisabeth himself.
Now, Carlo is at a Spanish monastery, near the grave of his grandfather, King Charles V. Monks come from the chapel. One of them approaches, and Carlo is shaken by this monk's resemblance to the dead King Charles.
Carlo is joined by his old friend Rodrigo, the Marquis of Posa. Rodrigo has just returned from Flanders, where the people are suffering under Spanish rule. Carlo admits to Rodrigo that he's in love with Elisabeth, who by now has married Phillip and is the queen of Spain. Rodrigo warns Carlo that being in love with his stepmother could prove dangerous, to say the least. He urges Carlo to come with him to Flanders, and fight for freedom and liberty, and the two declare their mutual loyalty in a stirring duet.
The second scene is outside the monastery. Elisabeth's ladies are waiting for her, along with the beautiful Princess Eboli. When Elizabeth arrives, Rodrigo appears and gives her a letter from her mother. In secret, he also hands her a note from Carlo. Rodrigo also talks with Eboli, who thinks that Carlo might just be in love with her. Rodrigo then tells Elisabeth that Carlo is suffering great emotional pain, and King Phillip is ignoring him. Carlo, Rodrigo says, would like an audience with Elisabeth.
Elisabeth agrees to see him. When he arrives, it's obvious that Eboli is not the one Carlo wants. He and Elisabeth are still in love, but they're in an untenable situation. Elisabeth is strong enough to endure the pain of their dilemma and carry on. Carlo is not; he breaks down, and openly declares his love for Elisabeth. She reminds him that if he wants them to be together, he'll have to kill his father, and then take his stepmother to the altar.
Carlo is distraught and leaves, and then King Phillip arrives with Rodrigo. As they talk, Phillip is clearly impressed with Rodrigo's loyalty and honesty. Taking him aside, the king confides in Rodrigo that he's suspicious of Elisabeth, and Phillip tactfully asks Rodrigo to keep an eye on her. He also gives the headstrong Rodrigo a stern warning to stay clear of the Grand Inquisitor.
ACT TWO begins at night, in the Queen's Gardens in Madrid. Carlo is there because he received an anonymous note from a woman, asking to meet with him at midnight. Princess Eboli appears. She still believes that Carlo may be in love with her, not knowing his feelings for Elisabeth. For this clandestine meeting, Eboli has come wearing a veil. Carlo assumes he's meeting Elisabeth, and again declares his love. Eboli is overjoyed, and removes the veil. Carlo's reaction tells Eboli all she needs to know: She's been mistaken for someone else. She also figures out who it is. Carlo admits that he's in love with his stepmother, the queen.
Eboli is shocked, and she's becoming more and more angry when Rodrigo shows up to try to rescue Carlo. Rodrigo tells Eboli that Carlo has been troubled, and that he's raving — she shouldn't take what he says literally. Eboli doesn't buy it. She threatens to tell the king everything, and Rodrigo promptly draws a knife, intending to kill her. At the last moment he changes his mind and says he has another plan to save Carlo. Rodrigo asks Carlo if he has any secret papers — maybe something pertaining to their mutual cause, the freedom of Flanders. Carlos does, and Rodrigo takes the documents, supposedly for safe-keeping.
The next scene is in a grand square, where crowds have gathered to honor King Phillip, and for an auto-da-fé — the ceremonial burning of heretics by the Inquisition. But the proceedings are interrupted when Carlo arrives. He's accompanied by six men from Flanders, who ask for the king's mercy. Phillip turns them down, and Carlo shocks the crowd by drawing his sword. Rodrigo steps between father and son, and tells Carlo to stand down. Feeling betrayed, Carlo gives up his sword to Rodrigo and is led away to prison. As a comforting angelic voice is heard, the ceremony continues and heretics are burned alive.
As ACT THREE begins, King Phillip is reflecting on his problems — and he's got plenty of them. He knows his wife Elisabeth doesn't really love him. Worse, he suspects that Elisabeth is in love with someone else. And now his son Carlo is in prison for treason.
The king also has another, even more threatening concern — the Grand Inquisitor. The Inquisitor may be old and blind, but he's still the most fearsome power in Spain. Phillip seeks the Inquisitor's advice about the rebellious Carlo. Should he let him escape into exile, or have him killed? The Inquisitor thinks Carlo should die. He says nothing — not even a father's love — is more important than faith.
The Inquisitor also has another demand. He says that Rodrigo must also be executed. He may be the king's best companion, but deep down, he's an even bigger traitor than Carlo. When Phillip refuses to hand Rodrigo over, the Inquisitor warns that even a king can suffer at the hands of the Inquisition.
When Phillip is alone again, Elisabeth enters, saying someone has stolen her jewelry box. She's surprised, and afraid, when the king produces the box from his desk. When she refuses to unlock it, he breaks it open. Inside, there's a picture of Carlo. Phillip accuses his wife of adultery, and she faints. Rodrigo and Eboli are called in to assist her. In a dramatic quartet, Rodrigo decides to take the situation into his own hands while Elisabeth feels abandoned by everyone, Eboli shows puzzling signs of guilt and Phillip realizes that he has accused Elisabeth falsely.
Phillip and Rodrigo leave the two women alone, and Eboli makes a confession. She was jealous over Carlo's love for Elisabeth, so she stole the jewelry box, and gave it to Phillip. She also admits to being the king's mistress, and begs for Elisabeth's mercy. Elisabeth gives her a choice between exile and a convent. Eboli decides on the convent — but first, she'll try to rescue Carlo.
ACT FOUR begins in Carlo's prison cell, where Rodrigo has come to visit. He tells Carlo not to worry. Rodrigo has allowed himself to be found holding the secret papers that Carlo gave him in back Act Two. This has revealed Rodrigo as the traitor, and now the king will want Rodrigo's head, not Carlo's. As they talk, two men enter quietly. One is with the Inquisition, and the other carries a rifle. From the shadows, Rodrigo is shot.
King Phillip enters with Princess Eboli and the Grand Inquisitor. Phillip means to free Carlo. When he finds Rodrigo dead, he sadly leans over the body and sings, "Who will give this man back to me?"
A mob assembles outside, demanding the traitorous Carlo be turned over to them. They turn quiet with fear when the Inquisitor shows his face. In the confusion, at Eboli's urging, Carlo escapes, and the scene ends.
The final scene is back at the monastery, where the opera began. Carlo is there, hiding near the tomb of King Charles V, his grandfather. Before he died, Rodrigo told Carlo that Elisabeth would meet him here, and she does. The two are saying goodbye when Phillip enters, with the Inquisitor. Phillip declares that he must do his duty, and he's about to hand Carlo over to the Inquisition to meet a grisly fate.
And then, we encounter this opera's famous — and famously perplexing — conclusion. What happens next depends on whose interpretation you believe, and what production you're watching. Some synopses keep it simple, saying something like, "the gates of the cloister open and Don Carlo is drawn inside to safety." In the Covent Garden production, an unidentified figure appears, dressed in a monk's robes but wearing an emperor's crown, and offers Carlo peace in heaven.
The opera's libretto is more problematic. It says the mysterious monk from Act One appears — now dressed as the dead Charles V. Actually, it says that this time, he actually is Charles V. Then, this supposedly dead king takes Carlo into the cloister, as everyone watches in amazement. It also says that Carlo is "bewildered" — which might be said of the audience, as well.
So what actually happens? Ultimately, it probably doesn't matter. The unfortunate Carlo has faced nearly every inner conflict imaginable — between love and loyalty; between duty and honor; between friendship and family; between faith and personal ideals. Unlike the other characters, Carlo never resigns himself to reality. He finds no way to cope with the complicated world that surrounds him. So, as the opera ends, the music lets us know that one way or another, Carlo is leaving that world altogether.
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