Elizabeth of Valois is promised in marriage to Don Carlos of Spain, as part of a peace treaty between the two kingdoms. They meet and fall in love – but no sooner have they declared their love than news comes that the terms of the treaty have changed: Elizabeth is to marry Carlos’s father Philip instead.
Elizabeth, now Carlos’s step-mother, loves him still, but is horrified by his declarations of love. The Grand Inquisitor urges King Philip to have Carlos executed for the sake of the faith and the stability of the kingdom. Carlos decides to flee. He meets Elizabeth one last time to bid farewell – but the Inquisitor’s forces descend upon them.