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.
