The story told by the film

The action takes place in France in the second half of the seventeenth century. France is in chaos, the starving citizens of the capital, Paris, require food to the king, who lives in his opulent palace away from the desperate requests of his subjects. The problem is that the King Louis (Leonardo DiCaprio) distributes the food to the army, so the peasant revolts are constant, but the musketeers, wich are the king's personal guard, broke them by the use of the force.Aramis (Jeremy Irons), a former Musketeer of the past king, proposed with his friends Athos (John Malkovich) and Porthos (Gerard Depardieu), a conspiracy to replace the king. Everyone thinks that it´s  crazy but Aramis knows the way. D'Artagnan (Gabriel Byrne) decides not to listen to Aramis and remains true and loyal to the King Louis and promises to protect him with his life.

One day, Athos, Porthos and Aramis infiltrate in the Bastille, with the plan to rescue a prisoner avoiding the guards. The prisoner was wearing a mask of solid iron that was covering all his face. After removing the mask, the prisoner said to the three musketeers that he used to be called Phillipe and he lived in the fields until the day that he was taken to prison. He spent 6 years inside prison without knowing the reasons and wearing the mask, he didn’t know where did he was born, or the true story of his past. 

Aramis told to Phillipe the whole truth, he told him that the queen mother Anne gave birth to two boys and they were twins. The king of France did not want that the two brothers  fight for the throne because that would divide France in two, so he decided to get rid of one of the twins, Phillip. The king ordered to take him to a house under extreme security, Queen Anne was told that one of the boys  died at the moment of the birth. When Louis ascended to the throne, he ordered to transfer  his brother Phillip to the prison, so that no one  know of his existence, and he also ordered to put him an iron mask. This is what Aramis told him. The whole truth about his past and its origin, and he confesses that he was the one who arrested him and put  the iron mask.  And asks him for  forgiveness. He told him that he would not be in peace, until Phillip will berestored to the throne.