It is the year 1662, and the French King Louis XIII has died, leaving the ruthless and arrogant Louis XIV as the only heir to the throne ... or is he? King Louis XIV (portrayed by Leonardo DiCaprio) has mercilessly locked his twin brother Philippe (also DiCaprio) into an iron mask and disposed of him in the Bastille, never to be seen or recognized again. To conceal the fact that two heirs were born, it was conceived that hiding the existence of one would eliminate future "problems". Their mother, Queen Anne, kept the secret known only to those very close to the King, including former Musketeer D'Artagnan (played by Gabriel Bryne), who displayed complete loyalty to the throne and obvious affection towards the Queen.

Phillippe, unaware that it was his evil brother who plotted against him or that he is also heir to the throne, spends his days in torturous desperation, while Louis' only thoughts are of his next amorous conquest and the constant "wars" to be fought. Louis' obvious unconcern is also apparent in his indifference to the suffering of his people, who starve and riot in the streets, while the young King lives a lavish lifestyle within the confines of his palace.

During a party given at the palace, Louis becomes enamored with Christine, the fiance of Raoul, son of Athos (brilliantly played by John Malkovich), once a member of the famed Three Musketeers. Louis immediately sends Raoul, a young soldier, off to fight on the front lines of war; Raoul is killed in battle, and Athos and two other Musketeers -- Porthos (Gerard Depardieu) and Aramis (Jeremy Irons) -- conspire to eliminate the heartless King. Though the threesome had retired years earlier after loyally serving King Louis XIII, they band together once again.

Aramis, now a Jesuit priest, hears of a rumor of a masked prisoner believed to be the imprisoned twin of Louis; they plot to free Philippe, and switch the brothers during a masquerade party at the palace. The only person still threatening their success is D'Artagnan, trainer of the Musketeers and head of the King's security. He is beginning to question Louis' ability to govern wisely; when he discovers the switch, he is reluctant to accuse his former Musketeers.

Louis is dragged off to the Bastille, and Philippe briefly assumes the role of King, until Louis reappears to regain his throne. Philippe is returned to prison, more secrets are revealed, and a swashbuckling finale pairs the Musketeers against the palace guards.