The Man in the Iron Mask (1998) on Netflix

In 1638 Queen Anne of France gives Birth to a boy, who will inherit the throne one day. But, only known to few, there is a twin brother born minutes later. 22 years later, King Louis XIV rules the country of France, but due to his lifestyle and the costs of war, people are forced to suffer hunger and poverty. Of the Musketeers, who had served his father Louis XIII faithfully, only one is still on the king's side: D'Artagnan, the Capitain of the king's guards. His three companions Aramis, Athos and Porthos have retired into civil life long ago. One day on a royal party, the young and unliked king gets a crush on Christine. This young lady is the fiancé of soldier Raoul, the son of Athos. Without further ado, the king has Raoul recruited and sent to war, so that he himself can take care of her. Soon Raoul dies in combat and the king has him out of the way. But Athos, Aramis and Porthos think of a plan to take revenge on the king. Aramis points out that there is a man in an iron mask in a very secure prison, and that this man's existance can solve all present problems. So they free the king's hidden twin brother, and tell him who he is, as he of course feels free of guilt and has no idea whatsoever why he was imprisoned for 6 years with this mask forged around his head. The plan now is to replace the evil king with his brother at a masquerade party held soon. Only D'Artagnan still is guarding the king and so he has to turn against his old friends...