We return to the fate of Monsieur Bonacieux, who has been taken to prison and questioned by the authorities about his wife. As it turns out, Bonacieux is much more concerned about his own avarice and safety than he is about his wife. He explains that his only interest in d'Artagnan was that he needed someone who could help him find his wife. When Athos is brought in, Bonacieux tells them that this man is not d'Artagnan. Bonacieux is then taken from the prison, placed in a carriage, and taken for a trip which he assumes is a ride to the gallows.
Later, Bonacieux is questioned by someone whom he discovers to be the powerful and imminent Cardinal Richelieu. While interrogating Bonacieux, the cardinal discovers that the houses which Bonacieux visited with his wife — houses which Constance Bonacieux had said were merchants' houses — are, in reality, the two houses where the duke of Buckingham and the queen's trusted friend, Madame de Chevreuse, have been hiding. When the cardinal calls in Count de Rochefort, Bonacieux immediately cries out that Rochefort is the man who abducted Constance.
Rochefort reports that the cardinal's spy in the queen's inner circle, Madame de Lannoy, has reported that the queen left her ladies-in-waiting and was gone for awhile. When she returned, she was carrying a rosewood box containing the diamond tags which the king gave her. She went into the antechamber and when she returned, she was empty-handed. The cardinal is certain that the duke of Buckingham has the coveted box and the diamond tags.
When Bonacieux is recalled and questioned further by the cardinal, the old man becomes putty in the hands of the honey-tongued cardinal; Bonacieux pledges everlasting loyalty to him. The cardinal then sends one of his men with a letter to be delivered to a woman in England, a certain Milady, who is to dance with Buckingham and secretly snip off two of the diamond tags that he will be wearing.
The next day, d'Artagnan tells Treville about Athos's mistaken arrest. Treville goes to see the king about Athos's arrest and discovers that the cardinal is already there; after much discussion, during which Treville vouches for the whereabouts of both Athos and d'Artagnan during the fracas with Bonacieux, the king and the cardinal both agree to let the matter rest.
Immediately after Treville leaves, the cardinal informs the king that Buckingham is in Paris. The king is certain that with the help of Madame de Chevreuse, the queen and the duke are seeing one another. When he hears that the queen has been writing letters that very morning, he is determined to have her searched and have the letters brought to him. He goes to see the queen and informs her that his chancellor, Sequier, will visit her soon and, at his command, will make a request of her. When Chancellor Seguier appears and searches the queen's room and desk and finds nothing, he prepares to search her person. The queen indignantly refuses, and he is about to use force when she reaches into her bosom and gives him a letter.
When the king opens the letter, he discovers that it is not a love letter to Buckingham; it is a political letter. The queen is asking her brother in Spain and her brother, the Emperor of Austria, to demand the dismissal of Cardinal Richelieu. The cardinal, upon reading the letter, cleverly offers to resign, but the king knows that he cannot manage France without the cardinal's powerful influence.
To make peace with the queen, the cardinal suggests that since the queen loves to dance, the king should give a big ball, and he tells the king that he should insist that the queen wear the diamond tags that he gave her as a present. On returning home, the cardinal hears from Milady that she has secured two diamond tags; she needs money to get to Paris, and as soon as she gets the money, she will be in Paris in four or five days. The cardinal then plots the date of the ball so that he might trap the queen.
In Chapter 13, we find out that Constance Bonacieux is only twenty-three years old; since she is married to a fifty-one-year-old stingy, selfish husband, she would naturally make a likely candidate for a love affair with d'Artagnan, especially since we also learn that her husband thinks of his love for his wife as being secondary to his love for money and influence. Bonacieux is thus an easy prey for the powerful cardinal, and he quickly becomes the cardinal's dupe. Later, when Constance asks him to do a service for the queen, he will not consent to it; thus, she turns to our hero, d'Artagnan, and asks him to perform this crucial deed for the queen.
In his questioning of Monsieur Bonacieux, the cardinal is seen to have an acute sense of the intrigues of the court. He knows that the duke of Buckingham is in Paris, and he is able to discover where both the duke and Madame de Chevreuse are staying — that is, in the houses that Constance Bonacieux often visited, pretending to her husband that she was visiting "tradesmen." Through his spies, the cardinal is able to deduce that the queen gave Buckingham the rosewood box containing the diamond tags. Knowing this, he requests the king to give a ball and demand that the queen wear the diamond tags. This demand, as we soon will see, will require d'Artagnan to go on his first adventure. He will have to get the diamond tags and return them to the queen before the date of the ball, a date which the cardinal sets as soon has he hears that his spy, Milady, has stolen two of the diamond tags — snipped them off while she was dancing with Buckingham.
In an earlier chapter, when d'Artagnan helped get a message to Monsieur de La Porte, the gentleman told d'Artagnan to find someone with a slow clock who could provide him with an alibi. D'Artagnan went to see Treville and reset his clock; now, when Treville has to give his word of honor that d'Artagnan was with him at a precise hour, he can do so — fully believing that he is telling the truth. Consequently, d'Artagnan is freed from all accusations by the cardinal.
Until Chapter 16, the reader might have wondered why the queen is such an enemy of the cardinal. It has been suggested that there are two reasons: (1) she is Spanish and Spain is France's enemy, and (2) she loves Buckingham, an Englishman, and England is an enemy of France. However, in Chapter 16, the real reason appears: ". . . the queen [Anne of Austria] was persecuted by the cardinal because he could not forgive her for having rejected his amorous advances."
Because of the cardinal's accusations about Anne's affair with Buckingham, the king is certain that his wife is untrue. He orders that her person be searched, and in those days, a gentleman's having his wife searched for a love letter was a dastardly thing, but a king having the queen searched was beyond comprehension. Thus, the cardinal, whose rumors cleverly prompt the search, now urges the king to be reconciled with the queen. Cunningly, he suggests a festive ball so that the queen will have to wear the diamond tags, which he feels sure are in the possession of the duke of Buckingham. Now the trap for the queen is set, and to counteract this trap, d'Artagnan will have to undertake the journey to recover the diamond tags and return them to the queen.