In 17th century France, the cruel King Louis XIV rules with an iron fist, causing widespread poverty and discontent. Unknown to many, he has a secret twin brother, Philippe, who has been imprisoned with an iron mask to keep his identity hidden. The aging Three Musketeers—Aramis, Porthos, and Athos—are disillusioned with the king's tyranny. Aramis, now a priest, reveals a shocking secret to his comrades: he is the leader of a secret society plotting to replace Louis with his more compassionate brother, believing that Philippe is the rightful king.

The Musketeers embark on a daring mission to free Philippe, seeking the help of the retired and disenchanted fourth Musketeer, D'Artagnan, who remains loyal to Louis out of a sense of duty. Despite personal conflicts and the challenges of confronting the king's formidable forces, the Musketeers must navigate a perilous path to rescue Philippe. Their plan not only involves extracting the unknown prince from his prison but also requires a dangerous charade to switch the siblings without arousing suspicion, all the while attempting to instill the values of the true Musketeers' motto: "All for one and one for all.