D'Artagnan, Gene Kelly, an inexperienced Gascon youth, travels to Paris with dreams of joining the elite King's Musketeers. On the road, he encounters a mysterious Lady De Winter, Lana Turner, at a roadside inn; when he challenges one of her escorts, she grows wary and has him knocked unconscious. His letter of introduction from his father to de Treville, the Musketeers' commander, is burned in the clash. When he wakes, he presses on toward the capital.
In Paris, he presents himself to de Treville, Reginald Owen, who recognizes the description of his assailant and makes him a cadet. The young Gascon quickly earns the attention—yet the ire—of three seasoned musketeers: Athos, Van Heflin, Porthos, Gig Young, and Aramis, Robert Coote, each challenging him to a duel. Before the duels can begin, Richelieu's guards move in to arrest the Musketeers. D'Artagnan joins their defense, helping to dispatch the attackers, and his bold stand earns him a place among the Musketeers.
Later, D'Artagnan rescues Constance Bonacieux, June Allyson, a confidante of Queen Anne Angela Lansbury, and he quickly finds himself drawn to her. The Queen—who has just received a matched set of twelve diamond studs from her husband, King Louis XIII, Frank Morgan—accidentally entrusts the jewels to her lover, the Duke of Buckingham, John Sutton, who is also a powerful political figure and Britain’s prime minister. Richelieu, sensing a political opportunity, schemes to provoke war with Britain by staging a ball in which the Queen will wear the diamonds.
D'Artagnan and his three companions volunteer to travel to Britain to retrieve the jewels, but along the way they are ambushed by Richelieu's men. One by one, the Musketeers are forced to stay behind to hold off their pursuers. Finally, only D'Artagnan and his servant Planchet, Keenan Wynn, reach the Duke. Richelieu has already sent the beautiful Countess de Winter to steal two of the studs. The Duke's jeweler manages quick replacements, and D'Artagnan races back to France. He arrives just in time to save the Queen from disgrace.
Admiring D'Artagnan's resourcefulness, Richelieu abducts Constance to enlist him in his service. He also assigns de Winter to help persuade the young man. D'Artagnan tries to learn where Constance is being held from the countess, but begins to fall under her spell instead. When Athos discovers that de Winter is actually his treacherous wife, he tells D'Artagnan, but is not believed. Then D'Artagnan notices a brand on her shoulder—the mark of a common criminal—just as Athos had warned him.
Fighting breaks out between Britain and France. The Queen frees Constance and sends her to Buckingham for safety. When the war goes against him, Richelieu commits de Winter to Britain to assassinate his foe. The Musketeers learn of the plot and send Planchet to warn the Duke. Athos confronts Milady and recovers the carte blanche as proof of Richelieu's treachery. De Winter is imprisoned by the Duke and placed in Constance's custody, but de Winter kills her first, then Buckingham.
D'Artagnan and Athos return to France to bring de Winter to justice. De Treville warns the Musketeers that she remains under Richelieu's protection.
Aramis recalls a conversation between her and Richelieu concerning the granting of a title and an estate near Lille. Caught at the estate, Athos's ancestral home, she begs for mercy, but finds none, even though her husband still loves her. Seeing no escape, she walks with dignity to her execution. The Musketeers are captured by Richelieu's men and returned to the royal court for judgment.
As Richelieu is about to have them sentenced to death, D'Artagnan produces the carte blanche. Richelieu is compelled to recommend to King Louis that he grant Aramis's wish to enter a monastery; Porthos, an introduction to a rich widow; Athos, the restoration of his title and lands; and D'Artagnan, a commission as a Musketeer and a mission to England, for > "the English lead too dull a life." <
The four depart triumphantly.
