D'Artagnan (Gene Kelly), an inexperienced Gascon youth, travels to Paris to join the elite King's Musketeers. On his way, he encounters a mysterious lady at a roadside inn. When he picks a fight with one of her escorts, she becomes suspicious and has him knocked unconscious. His letter of introduction from his father to de Treville (Reginald Owen), the commander of the Musketeers, is burned. When he awakens, he continues on to the city.In Paris, he nevertheless presents himself to de Treville, who recognizes d'Artagnan's description of one of his assailants and, saying "A man is sometimes known by the enemies he makes," makes him a cadet. The young Gascon spots the very man and in his haste to confront him, annoys three of the most skillful Musketeers: Athos (Van Heflin), Porthos (Gig Young) and Aramis (Robert Coote). Each challenges him to a duel. At the appointed place, the master swordsmen are amused by the newcomer's audacity. Before they can begin however, they are interrupted by Richelieu's men, who try to arrest the Musketeers. Outraged that the three are outnumbered, d'Artagnan joins them in dispatching their foes, displaying his superb swordsmanship in the process. As a result, he is welcomed into their ranks.Later, d'Artagnan rescues (and falls in love with) Constance Bonacieux (June Allyson), a confidante of Queen Anne (Angela Lansbury). The queen had been given a matched set of diamond studs by her husband, King Louis XIII (Frank Morgan). Foolishly, she gives them to her lover, the Duke of Buckingham (John Sutton), who is also the Prime Minister of Great Britain. Knowing of the queen's indiscretion, Richelieu (Vincent Price) sees a way to persuade the king to go to war with Britain. Richelieu arranges a ball and suggests to Louis that his wife wear the diamonds.D'Artagnan and his three friends 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) are left to reach the duke. However, Richelieu had already sent the beautiful Milady, Countess de Winter (Lana Turner) to work her wiles on the duke and steal two of the studs. Fortunately, the duke's jeweler is able to make replacements quickly and d'Artagnan races back to France. He arrives just in time to save the queen from disgrace.Admiring d'Artagnan's resourcefulness, Richelieu has Constance abducted in an attempt 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 de Winter, but begins to fall under her spell instead. When Athos discovers that Milady is actually his treacherous wife, he tries to warn d'Artagnan, but is not believed. Then d'Artagnan finds out that Athos was telling the truth; he sees a brand on her shoulder, the mark of a common criminal.Fighting breaks out between Britain and France. The queen succeeds in freeing Constance and sends her to Buckingham for safety. When the war goes against him, Richelieu gives de Winter a carte blanche and sends her to Britain to assassinate his foe. The Musketeers learn of the plot and send Planchet to warn the duke. Athos confronts De Winter and recovers the carte blanche as proof of Richelieu's treachery. De Winter is imprisoned and placed in the custody of Constance, but when the latter lets down her guard, de Winter kills her, then Buckingham.Caught once again by the Musketeers at the ancestral home of Athos, she begs for mercy, but finds none, even though her husband still loves her despite her many crimes. Seeing this, she calms herself and walks with dignity to her execution. The Musketeers are ambushed by Richelieu's men and captured, but as he is about to have them sentenced to death by the king, d'Artagnan produces the carte blanche, and Richelieu is compelled to grant the Musketeers a comfortable retirement and d'Artagnan a commission as a Musketeer.
Output: Question: What do the Three Musketeers decide to try to prove the Cardinal guilty of?, Answer: treason