Love at first sight: Carlos, the Infante of Spain, has travelled to France to meet the woman chosen as his bride, Élisabeth de Valois. Both immediately fall passionately in love with each other, but are dismayed to learn that it is now no longer Carlos but his father Philippe II who is to marry the princess. For the sake of peace between Spain and France, Élisabeth agrees to the match. Broken-hearted, Carlos confides in his friend Rodrigue, the Marquis de Posa, who has just returned from Flanders. The marquis asks Carlos to come to the aid of this oppressed and exploited country, saying it would help him forget his love of Élisabeth. The two men swear eternal friendship. With Posa’s help, Carlos and Élisabeth meet again. Their old passion reignites, but Élisabeth reminds him of her marital obligations. Disappointed and hurt, Carlos storms off. Philippe, impressed and fascinated by Posa’s youthful impetuousness and honesty, places his trust in him even though the marquis is striving for Flanders’ freedom and independence. Philippe feels an emotional bond with him that he has never been able to feel towards his son and wife. He asks Posa to keep an eye on Carlos and Élisabeth. Carlos inadvertently reveals his love of the queen to Princess Eboli, who is secretly in love with him. She now begins to plan retaliation. When Carlos tries to persuade his father to appoint him governor of Flanders, the king angrily rejects the idea. The dispute threatens to escalate. Posa hurries to Philippe’s aid and disarms the stunned Carlos. In the meantime, Eboli starts her revenge by sending a secret casket belonging to Élisabeth to the king. Inside he finds a portrait of his son. Deeply hurt he accuses his wife of adultery and has Carlos locked up. When Posa visits him in prison, the marquis is murdered by a sniper’s bullet. When he states that not he, Carlos, but Posa was part of the uprising in Flanders the infante is released. Carlos is now determined to forsake Élisabeth and to fight for Posa’s ideals, and therefore against his father, in Flanders.