We begin the novel in France, where a young man from Gascony (a city in France) named D’Artagnan sets out for Paris with three gifts from his father: fifteen crowns, a horse, and a letter of introduction to M. de Tréville, who is a Very Important Person since he commands the King’s Musketeers.
D’Artagnan hopes to become a Musketeer one day, but he doesn’t have much going for him except for his training as a gentleman, which means, effectively, that he can handle a sword and be polite about it. His pluck, determination, and good manners lead him to become BFF with three Musketeers named Porthos, Athos, and Aramis, and also lead him to fall in love with the beautiful Constance Bonacieux.
Meanwhile, the Duke of Buckingham (i.e., only the biggest deal in England) is wooing Queen Anne of France. Since they can’t exactly be public about their feelings for each other (she’s married to the King of France, after all), Anne gives him some diamond studs as a consolation prize during his trip back to England.
Enter the nefarious Cardinal, who is the biggest deal in France—yes, even bigger than the King. The Cardinal, who’s still angry that Anne burned his declarations of love some time back, wants her to get in trouble with her husband. He knows, through his spies, that Anne gave the diamonds to Buckingham. He suggests that the King throw a fête (party) and require Anne to wear the diamonds (which were originally a gift from the King, which means that Anne re-gifted a gift from her husband to her lover—classy). As soon as Anne finds out about this fête and this requirement, she bursts into tears.
Enter Madame Bonacieux, who promises that she will find someone to help the Queen regain the diamond studs in time for the ball. This someone turns out to be D’Artagnan, who’s just tripping all over himself to get in her good graces. He takes his buddies Athos, Porthos, and Aramis to England to retrieve the studs, but one by one each of them is detained on the road. D’Artagnan makes it to London alone and meets with the Duke. He finds the studs, but two are missing.
To solve the problem, the Duke has two new ones whipped up and blocks any ship from leaving England to ensure that the missing studs don’t make it to Paris. This, of course, means that he’s declared war on France, but obviously the honor of his beloved is much more important. D’Artagnan makes it back to Paris in time to save the Queen.
Now Madame Bonacieux is ready to get busy with D’Artagnan—she makes a date with him for ten o’clock at a little pavilion. When he shows up, however, D’Artagnan waits to no avail. It turns out Madame Bonacieux was kidnapped. The plot thickens.
D’Artagnan next skips town with Planchet (his trusty lackey) to find out what happened to his friends. They find Porthos and Aramis at two inns along the way, but both are still too wounded for travel and so D’Artagnan leaves them both horses and presses on to find Athos. D’Artagnan finds Athos at another inn down the road, and the two friends reunite over several bottles of wine. While intoxicated, Athos tells D’Artagnan how a woman once ruined a "friend" of his. This friend was a nobleman who, many years ago, married a beautiful woman against the wishes of his family, only to discover she was a branded criminal. His heart has been broken ever since (foreshadowing!).
After a few bumps in the road (Athos gambles away two of the horses, and Porthos and Aramis lose theirs as well), the four return to Paris and D’Artagnan finds out that the King is recommending him to become a Musketeer. This joy is short-lived, however, as all the men must somehow gain enough money to outfit themselves properly for war. Porthos and Aramis appeal to their mistresses, and D’Artagnan sleeps with a noblewoman in return for a valuable ring (that, we find out, once belonged to Athos) and the two sell it to split the money. The friends find themselves well-funded and ready for war—all is well in the world, right?
Wrong. Trouble is, D’Artagnan didn’t just sleep with any old woman—he slept with Milady, who is an agent of the Cardinal. Not only did he sleep with her, he uncovered her secret: she has a fleur-de-lis branded into her shoulder, marking her a criminal. As a result, Milady sends two assassins after him and then follows that up with some poisoned wine. Our young hero barely escapes both times.
Meanwhile, Athos, Porthos, and Aramis eavesdrop on a conversation between the Cardinal and Milady. She is charged with going to England and persuading someone to assassinate the Duke of Buckingham. In return, she wants D’Artagnan dead. Needless to say, D’Artagnan doesn’t take this bit of information well.
The friends decide to send a lackey to Tours (a city in France) with a letter warning the Queen of the plot against Buckingham, and to send another lackey to England to warn Milady’s brother-in-law about her arrival. When Milady arrives in England, therefore, she is escorted to a snug room in a castle above a cliff. The room has lots of bars on it. However, Milady soon manages to corrupt her jailer and convince him that she is a pure and innocent woman ruined by the Duke of Buckingham. Her jailer (John Felton) sets her free, puts her on a ship, and then he stabs the Duke to death. Milady sets sail for France, destined for a convent where Constance Bonacieux is hiding.
D’Artagnan is overjoyed when word comes from the Queen that he can meet Constance and take her away from the convent. He arrives moments too late, however, as Milady exacts her revenge on D’Artagnan by fatally poisoning Constance. She dies in D’Artagnan’s arms. The four friends track Milady down and bring her to a trial, where we hear the full extent of her crimes. The sentence pronounced upon her is death. She is executed.
As the friends head back to war, the Cardinal requests to see D’Artagnan. Even though D’Artagnan is afraid he’ll be sentenced to death, he bravely heads to the meeting. He confesses to the Cardinal that Milady is dead, but since the Cardinal was only going to kill D’Artagnan as a favor to Milady, he changes his mind and gives D’Artagnan a commission as a lieutenant in the Musketeers.
D’Artagnan is overcome and protests that one of his friends should take it. He offers it to them each in turn, but they refuse. Athos doesn’t want it, Aramis is going to become a priest, and Porthos is going to marry a rich woman. Monsieur Bonacieux disappears under mysterious circumstances, and there the novel ends.