D’Artagnan arrives in Paris and, seeking to join the king’s musketeers, goes to see their captain, Tréville. In his haste he offends three of the best musketeers—Porthos, Athos, and Aramis—and challenges each to a duel that afternoon. All three musketeers arrive at the appointed location at the same time to duel with D’Artagnan. However, they are interrupted by Cardinal Richelieu’s guards. The musketeers plus D’Artagnan happily engage the guards and beat them soundly, and D’Artagnan is accepted as a friend and a good fighting companion.
D’Artagnan settles into his new life, hoping to soon become a musketeer, and rents an apartment above the shop of Monsieur Bonacieux. However, he finds himself in the middle of the foulest plot in France.
Cardinal Richelieu and the evil Milady de Winter are trying to discredit Queen Anne in the eyes of King Louis XIII and the country, thus ultimately giving Richelieu more power over the king. The queen is trying to repel the amorous advances of the English duke of Buckingham. And Constance, Bonacieux’s wife, has become the trusted messenger between the queen and Buckingham, as well as a victim of Richilieu who sees her as an avenue to expose the queen. D’Artagnan becomes involved when he meets Milady and is smitten by her charms and when Constance asks him for help and he is smitten again.
About this time, Athos, to distract D’Artagnan from thoughts of Milady and Constance, tells him about a man who married a woman whom he later learned was a convicted thief. He believes the evil woman is now dead. The man is, of course, Athos, but he doesn’t tell D’Artagnan that.
Next, Richilieu, through Milady and his henchman Rochefort, trick Buckingham into coming to Paris. Buckingham goes to the queen’s chambers, but she refuses his love. He asks for a remembrance, and the queen gives him twelve diamond studs; then, with Constance and D’Artagnan’s help, he leaves England safely. Even though Richilieu doesn’t trap Buckingham this time, his spies inform him of these happenings.
Still plotting to trap Queen Anne, Richelieu suggests King Louis give a ball ten days hence at which the queen can wear the diamond studs the king has given her, and he sends Milady to London to steal the jewels from Buckingham. Learning of the ball,the queen writes to Buckingham to return the jewels to her, and she asks D’Artagnan to take the letter to England. D’Artagnan enlists the help of Athos, Porthos, and Aramis; and the four race off to England.
The four friends soon are separated, but D’Artagnan eventually arrives in London; however, when Buckingham gives D’Artagnan the diamond studs, they discover two are missing. Buckingham realizes that Milady, who arrived earlier, has managed to steal them, but he commands a jeweler to make duplicates, and arranges to hand over the complete set to D’Artagnan at the dock in the morning.
The next morning, Milady arrives at the dock first and tries to get the jewels, but Buckingham will not give her the studs, so Milady stabs him. D’Artagnan arrives, preventing her from stealing the jewels, but not in time to prevent Buckingham’s death.
Milady hurries back to Paris ahead of D’Artagnan and tells Richelieu of her partial success: she believes that, even if D’Artagnan brings the jewels, two will be missing. The king arrives at the ball and notes the absence of the queen’s diamond studs. He demands she get them, saying he will come back when she is properly dressed. With time running out, D’Artagnan finally arrives and gives the queen the jewels—the original ten, plus the two new ones. The king returns, and the queen is happily wearing all twelve diamonds.
In the meantime, the queen has sent Constance to a convent where she can be safe from the cardinal. She tells this to D’Artagnan, but Milady overhears and hurries off to seek revenge. D’Artagnan gathers up his three friends, and the race is on again. Milady, disguised, arrives at the convent before the musketeers and secretly pours poison into a glass of wine and urges Constance to drink. The four friends arrive as Milady rushes out, and Constance dies in D’Artagnan’s arms.
Finally, the four catch up with Milady. They accuse her of her various crimes and pass sentence of death—and Athos recognizes her as his long-ago wife whom he thought was dead. However, Milady cheats them of revenge by stabbing herself. The musketeers kneel and ask for God’s forgiveness on all of them.