Crown Prince Don Carlos reunites with reformer Marquis Posa, who urges him to govern Flanders and broker peace. Carlos confesses forbidden love for his stepmother, Queen Elisabeth, who bids him serve Spain instead. Intrigue by the jealous Princess Eboli and Duke of Alba convinces King Philip II of betrayal. Posa sacrifices himself to save Carlos, but the king ultimately imprisons his son and hands him over to the Inquisition.