In 1625, young-but-skilled fencer d'Artagnan sets off for Paris, France in hopes to follow in his murdered father's footsteps and become a member of the musketeers: the personal guard of the King of France. He is pursued by Gérard and his brothers, accused of blemishing their sister's honor. Meanwhile, Captain Rochefort of the Cardinal's Guards disbands the musketeers per the orders of Cardinal Richelieu, the King's Minister, ostensibly to help fight in an impending war with England. Rochefort informs Richelieu that three musketeers refused to relinquish their duties: Athos, Porthos, and Aramis.
In Paris, d'Artagnan "rescues" the queen's handmaidens from their own bodyguards, and after a scolding, takes a liking to one, Constance. In the city d'Artagnan encounters each of the Three Musketeers separately, unaware of their identities or association, resulting in a separate duel arranged with each. At the arranged location, Athos, Porthos and Aramis reveal themselves as musketeers. Before they can duel, a Captain of the Cardinal's Guard arrives with four other guards to arrest the musketeers; although d'Artagnan is not sought by the guards, he allies with the musketeers during the skirmish. The Musketeers kill four guards, while d'Artagnan outduels the Captain, who falls to his death. Impressed but displeased at d'Artagnan's involvement, the musketeers leave d'Artagnan behind after encouraging him to flee and maintain his innocence. When more of the Cardinal's Guards arrive, d'Artagnan is captured.
D'Artagnan escapes his cell and eavesdrops on a conversation between Richelieu and the mysterious Milady de Winter, where the Cardinal plots to supplant the King, tasking Milady with delivering a secret treaty to England's Duke of Buckingham. D'Artagnan is caught by Rochefort without having seen Milady's face. Richelieu orders him executed for refusing to give up the musketeers' location, but he is saved by the musketeers. As they flee, d'Artagnan reveals Richelieu's plans; they decide to intercept the spy at Calais and retrieve the treaty to prove Richelieu's guilt.
During a skirmish, the party splits up; d'Artagnan rides ahead to Calais, but passes out from exhaustion and is found by Milady de Winter. He wakes in a bed stripped of his weapons and clothes as Milady tries to seduce him. d'Artagnan speaks openly of his plans, not knowing she is the spy. She attempts to kill him, but he convinces her to keep him alive. As her party boards the boat to England, they are confronted by the musketeers. Milady attempts to run away but is stopped by Athos, who is revealed to have been her first husband, but betrayed her to the authorities when he found out she was branded a murderer. The musketeers retrieve the treaty and Milady is sentenced to death for the murder of her second husband, Lord de Winter; just before she is executed, Athos begs her forgiveness. Moved, she reveals the Cardinal's plans to assassinate the king at his birthday celebration, then throws herself off a cliff to her death.
Athos, Porthos and Aramis send missives to rally the rest of the musketeers. Richelieu and Rochefort hired a sharpshooter; during the assembly, d'Artagnan interrupts the sniper's shot, which narrowly misses the king. The musketeers reveal themselves, and Richelieu blames them for the attempted assassination.
As the three face off with the Cardinal's guards, men rush to their aid and reveal themselves as musketeers. The two forces battle as Richelieu takes the king and queen hostage, shooting Aramis in the chest before fleeing to the dungeon with Athos and Porthos in pursuit. d'Artagnan duels Rochefort and is disarmed; as Rochefort gloats about having killed d'Artagnan's father, Constance retrieves and throws him his sword and d'Artagnan promptly kills Rochefort.
Athos and Porthos arrive just as Richelieu's boat leaves on an underground river, with Richelieu vowing to return. The boatman then reveals himself as Aramis, his crucifix having stopped the bullet. Aramis moves to apprehend the Cardinal, but King Louis punches Richelieu himself, knocking him into the river.
The musketeers are reinstated by the king, and d'Artagnan is offered anything he wants; he chooses to serve Louis as a musketeer. Outside the musketeer headquarters, Gérard and his brothers arrive and challenge d'Artagnan to a duel; Porthos reminds him that musketeers not only protect King and country, but also each other. Gérard and his brothers are then chased off by the entire musketeer division.
