The king wonders briefly why the cardinal is so insistent that the queen wear the diamond tags, but he nevertheless tells the queen about his plans for the ball and instructs her to wear the diamond tags. On further questioning, the queen learns that the idea of having a ball was the cardinal's idea; furthermore, it was the cardinal who suggested that she wear the diamond tags.
After the king leaves, the queen is filled with fear. Suddenly, Constance Bonacieux enters from the closet and reveals that she knows the entire story; furthermore, she promises that she will find someone to go to the duke of Buckingham and retrieve the diamond tags. The queen reminds Constance that a letter would have to accompany the messenger and, if intercepted, she (the queen) would be ruined — divorced and exiled. Constance, not knowing of her own husband's allegiance to the cardinal, swears that her husband will do anything for her. Relieved, the queen gives Constance a jewel to sell in order to defray the expenses of the journey.
At home, Constance discovers that her husband has become an ardent cardinalist and will have nothing to do with her intrigues: "Your queen is a treacherous Spanish woman, and whatever the cardinal does is right," he says. Constance also discovers that her husband is in league with Count de Rochefort, even though he knows that Rochefort is the person who abducted Constance. Monsieur Bonacieux leaves and Constance is certain that he will betray her.
D'Artagnan overhears the entire conversation between husband and wife, and later he is delighted to assert that her husband is a wretch. He then offers himself at her service. When Constance is reluctant to tell d'Artagnan all of the details about the mission, he reminds her that she was about to tell her traitorous husband everything, and furthermore, d'Artagnan loves her more than her husband does.
Constance relents and tells him all about the secret mission, and d'Artagnan promises to obtain a leave of absence and be on his way to London. Constance suddenly remembers the three hundred pistoles that the cardinal gave her husband, and she gives the money to d'Artagnan for the journey. D'Artagnan is delighted: "It will be twice as amusing to save the queen with His Eminence's [the cardinal's] money."
At that moment, they hear her husband returning with someone. D'Artagnan recognizes the person as "the man from Meung," and he is ready to attack him when Constance stops him because of his duty to the queen; in other words, first things first. They listen and overhear her husband's plan to supposedly relent and agree to go on the errand for his wife; then, after he has the queen's letter to Buckingham, he will take it to the cardinal.
On his way to Treville's house, d'Artagnan wonders if he should tell Treville about the secret mission; interestingly, Treville tells d'Artagnan to keep the details of the mission secret and, instead, to ask for whatever favors he needs. D'Artagnan says that the cardinal will do anything to keep him from getting to London, and Treville suggests that at least four people should go on the journey so that one of them might succeed in actually getting there. D'Artagnan says that Athos, Porthos, and Aramis will accompany him without demanding to know the nature of the secret mission. Accordingly, Treville writes out passes, and d'Artagnan goes to each of the musketeers and tells them to get ready for the trip. They discuss several tactics for successfully accomplishing the mission, but d'Artagnan tells them that they must all go together, not in separate directions, because if one of them is killed, the others can make certain that the letter is finally delivered to London. They agree and begin to make preparations to leave.
When the queen is instructed to wear her diamond tags to the ball, she is also told that it was the cardinal who proposed having the ball. Constance quickly realizes that the idea of her wearing the diamond tags was also the cardinal's idea. As a consequence, she knows that the cardinal has a spy among one of her ladies-in-waiting, but she does not know which one. Therefore, when Constance Bonacieux appears from the closet, where she has been tending to the linen, she could have overheard the conversation between the queen and the king; therefore, the queen is not sure, at first, if she can trust Constance. But after Constance's protestations of loyalty and her reminder that she is the person who brought Buckingham to her, the queen is finally convinced that she can trust Constance. Now we can see that these earlier episodes function as a basis for Constance's loyalty and are proof that the cardinal is indeed a powerful enemy of the queen.
In a similar way, we can now look back at other scenes. For example, when we read that Constance Bonacieux discovered that her husband was a cardinalist — totally devoted to and committed to the cardinal — we realize now how the cardinal used his interview with Constance's stupid husband in order to gain another loyal adherent. Dumas closes Chapter 18 with a brilliant stroke of irony: the old miser Bonacieux is howling for his missing money. D'Artagnan's trip to London will be financed by money which the cardinal gave to Bonacieux.
Clearly, Dumas delights in d'Artagnan's heroics. In the scene where Constance is in despair, fearing that the mission for the queen is doomed to fail, Dumas uses the romantic device of having d'Artagnan overhear the entire conversation between Constance and her husband; then, suddenly and romantically, d'Artagnan presents himself as her rescuer and savior. The queen's honor can be preserved.
Note too how Dumas uses a combination of circumstances in order for d'Artagnan to be fully characterized as the romantic hero: he is in the right place at the right time and overhears the right kind of intrigue so that he can become involved in the affairs of great people. Dumas also stresses that it is d'Artagnan's plan for the mission that the older, more experienced musketeers finally accept. D'Artagnan is younger than the other men, but already he seems to have a natural talent for intrigue and adventure; in fact, Buckingham will later marvel at d'Artagnan's being so young, yet so dashing, brave, and inventive.