Don Carlos was first performed in French. Later, an Italian version was prepared and it is referred to as Don Carlo
In the first half of the 20th century, Don Carlos was rarely performed, but in the postware period it became part of the standard operatic repertoire. There are a number of recordings of both the French and Italian versions, and it is regularly performed.
ACT I. Forest of Fontainbleau. Don Carlos, son of Philip II and Crown Prince of Spain, is the affianced lover of the beautiful Elizabeth of Valois, daughter of Henry II. Don Carlos and Elizabeth meet for the first time, and Don Carlos pretends that he is only a messenger from the "Infant" with a gift for her. He tells her of Don Carlos’s admiration for her. She opens the package and finds a miniature of Don Carlos, and at once recognises him as the original. They fall deeply and passionately in love with each other. A messenger arrives with the news that she is to marry, for reasons of state, Phillip II, not his son, Don Carlos. The lovers are in despair.
ACT II. The royal marriage is duly solemnised; but Don Carlos can not overcome his love for Elizabeth. His friend, Rodrigo, advises him to go to Flanders, to forget his passion; and Don Carlos asks Elizabeth to gain the requisite permission from the King. Their interview serves to re-awaken the intensity of their love; and Don Carlos clasps her in his arms forgetful of all else, and then flees from the scene.
ACT III. Part I. The Queen’s Garden. During a carnival Don Carlos meets the Princess Eboli, and mistaking her for Elizabeth, tells her of his love. The Princess loves him herself, and she joyously removes her mask. When she learns of her mistake, filled with jealousy, she threatens to reveal Don Carlos’s love for Elizabeth to the King. Part II. A large square before Nostra Donna d’Atocha, with a funeral pile. The bells are ringing joyously. The Court and the Queen, and later the King, enter, as for a holiday. Don Carlos appears at the head of a delegation of Flemings and begs for mercy for them. The King refuses, and Don Carlos, drawing his sword, vows to be their savior. The King orders him disarmed, but everyone is afraid to attempt it until Rodrigo asks him for his sword and Don Carlos yields. The funeral pile is lit and the joyous song of the Inquisitors is heard.
ACT IV. Part I. In the King’s Library. The Grand Inquisitor convinces the king that Don Carlos must be imprisoned. The Princess Eboli arouses his jealousy by telling him of the love between his son and his wife. Part II. The prison of Don Carlos. Rodrigo comes to visit his friend and is shot by unknown men, by order of the Inquisition which fear his enlightened spirit. Part III. The Cloisters of the Convent of St. Just. Don Carlos has been freed at the demand of the populace and hastens to the Convent for a last farewell with Elizabeth. Elizabeth is exhorting him to help Flanders, and so distract his mind from his own sorrows, when the King suddenly arrives, having heard of their clandestine meeting, and delivers his son over to the Inquisition.
References and external links: Plot taken from The Opera Goer's Complete Guide by Leo Melitz, 1921 version.