Don Carlo, heir to the Spanish throne, was supposed to marry the French princess Elisabeth.
After meeting for the first time, the two are in love. Surprisingly, the Spanish King Philip II announces that he wants to marry Elisabeth himself. Elisabeth complies for reasons of state. The liberal Marquis Posa advises his desperate friend Don Carlo to become politically active for a better future for Flanders. However, this puts Don Carlo and Posa in dangerous opposition to the overpowering Grand Inquisitor.