On the way back to La Rochelle, the King and his Musketeers are all in a dreary mood. The King, who gets bored easily, is not eager to return to the drudgery of the slow-paced siege. The Musketeers who guard him take their cues from his behavior. Even the few among them who tend toward natural cheerfulness—such as Athos, Porthos, Aramis, and d’Artagnan—seem unhappy.

When the King stops to hunt magpies one day, d’Artagnan and his friends go to a tavern for a drink. Shortly after they sit down, a dark-haired man approaches. It is none other than the man of Meung, also known as the Comte de Rochefort. D’Artagnan leaps to his feet and demands a duel immediately, but Rochefort has other ideas. He has come to place d’Artagnan under arrest, by order of Cardinal Richelieu.

D’Artagnan does not find it particularly surprising that the Cardinal wants to arrest him, so he makes no objection and hands over his sword. The Musketeers are on their way to La Rochelle regardless, so d'Artagnan solemnly promises to report to the Cardinal upon arrival. He keeps this promise, and his friends accompany him. They stand outside, ready to fight in his defense if necessary.

When d’Artagnan enters the Cardinal’s office, he thinks that all may be lost. Even his friends’ fighting prowess is no match for the Cardinal’s power. Nevertheless, he defends himself bravely. When the Cardinal explains that Milady has accused d’Artagnan of several serious crimes, d’Artagnan replies that his accuser is not trustworthy. When the Cardinal asks for an explanation, d’Artagnan calmly explains that he and the other Musketeers put Milady on trial, found her guilty, and had her executed. The Cardinal says that they had no right to do such a thing, but d’Artagnan calmly produces his pardon, the Cardinal’s letter:

It is by my order and for the service of the State that the bearer of this note has done what he has done.

As both men know, these words were written to pardon the murderer of d’Artagnan rather than the murderer of Milady. Nevertheless, the Cardinal falls into a thoughtful silence. He is not sorry to have lost Milady, especially if she was really a murderer and an adulterer. He tears up the pardon and gets out a pen to write something else.

Watching the Cardinal, d’Artagnan thinks he is about to be sent off to prison, or even to the gallows. Instead, the Cardinal gives him a paper authorizing a promotion to lieutenant among the Musketeers. He tells d’Artagnan to take the promotion himself or give it away to one of his friends, whichever he pleases.

That evening, d’Artagnan offers the promotion to Athos, who refuses it, saying that he would rather see d’Artagnan keep it. D’Artagnan offers the position to Porthos and Aramis, who each refuse as well, saying that they have other plans. Thus, as The Three Musketeers ends, d'Artagnan becomes a lieutenant among the Musketeers.

Epilogue

When La Rochelle surrenders, the King and his Musketeers return home to Paris victorious. D’Artagnan takes his role as a lieutenant, and Athos serves beneath him for several more years. Meanwhile, Porthos leaves the military life to marry the newly widowed Madame Coquenard, and Aramis finally becomes a priest.

The four heroes’ lackeys do well for themselves as well. Planchet joins the guards in the Piedmont Regiment and becomes a sergeant. The other three follow their masters, Mousqueton and Grimaud continuing in their roles as servants, and Bazin joining Aramis’s monastery.

Over the next few years, d’Artagnan and Rochefort duel three times. D’Artagnan wounds Rochefort on all three occasions, but the two men gradually lose their dislike for one another. By the end of the final duel, they make a pact of friendship.

Monsieur Bonacieux never finds out what happened to his wife, but he does not particularly care. He does hope to receive further benefit from his relationship with the Cardinal, who finds this annoying and has Monsieur Bonacieux sent to prison for the rest of his days.