The term "mousetrap" is explained as being a method whereby the police trap friends and/or associates of a person who has been arrested for political reasons. Here, the authorities have placed four guards at Monsieur Bonacieux's house, and they plan to arrest anyone who knocks. Meanwhile, upstairs, d'Artagnan has removed most of the first section of the flooring in his apartment so that he can hear the entire proceedings. When he hears the guards manhandling Constance Bonacieux, he sends his servant, Planchet, to enlist the aid of the three musketeers, and grabbing his sword, he flies to Constance's rescue. Only one of the guards is armed, and after a short time, d'Artagnan is able to drive all four men from the premises in a manner so dashing and thrilling that Constance is marvelously impressed and eternally grateful.
Constance Bonacieux turns out to be young (in her early twenties), charming, and beautiful. When she describes the man who abducted her, d'Artagnan recognizes him as "the man from Meung." She tells d'Artagnan about her escape: she was left alone, so she immediately tied some sheets together and let herself down from a window. She feels so deeply grateful to d'Artagnan that she entrusts him with a secret password which will gain him entrance into the palace to see Monsieur de La Porte, whom he is to send to her. When d'Artagnan delivers the message to La Porte, the gentleman advises him to find someone whose clock is slow and go there and establish an alibi.
Afterward, d'Artagnan daydreams about a romantic love affair with Constance Bonacieux, and while wandering idly through the Paris streets, he finds himself outside Aramis's house, where he sees a lady in a cloak knocking at what appears to be Aramis's window. He sees the woman talking to another woman, and when she leaves, he discovers that it is Constance Bonacieux. He follows her and accosts her. She denies knowing Aramis, and when she refuses to reveal the secret of her mission, d'Artagnan offers to escort her to her destination. She permits him to do so on condition that he leave and not follow her. D'Artagnan promises and returns home, where he learns that Athos has been arrested by authorities who thought that they were arresting d'Artagnan.
D'Artagnan sets out for Treville's house to tell him about the arrest and other events. On a bridge, he sees two figures — one is dressed exactly like Constance Bonacieux and the other is in a musketeer's uniform; his appearance resembles Aramis. When d'Artagnan brashly stops them, calling out Aramis's name, he discovers that Constance Bonacieux is escorting the duke of Buckingham to the Louvre Palace. D'Artagnan is pleased to escort them safely to the palace.
At the palace, Constance leads the duke through a series of corridors and leaves him in a private anteroom. Soon, Anne of Austria, the queen of France, appears, and the duke makes his protestations of love to her, but she continually and sadly rejects his overtures, even though she is obviously in love with him. As a parting gift for him, she goes to her chambers and returns with a rosewood box as a token of her love. Inside the rosewood box are twelve diamond tags, or studs (button-like ornaments).
Here, as part of the novel of intrigue, we are introduced to the villainous "authorities" who set a trap and arrest anyone — innocent or guilty — who enters the "mousetrap." In modern terms, this is similar to police entrapment, a technique whereby the police use an officer to trap someone into violating a law so that the police can arrest that person. It is by this method that d'Artagnan meets Constance Bonacieux, who becomes his first love.
Constance Bonacieux's escape from her captors (by tying sheets together and letting herself down from a window) and d'Artagnan's rescue of her are in the best swashbuckling, romantic tradition, as is the scene where the four guards battle against d'Artagnan and d'Artagnan overcomes these odds and rescues the fair damsel in distress.
Also in the tradition of the troubadors and other devoted cavaliers who love for-the-sake-of-love, d'Artagnan immediately falls in love with Constance Bonacieux; she will be d'Artagnan's beloved for whom he will perform valorous deeds. His relationship with Constance Bonacieux will, of course, eventually cause him to volunteer to perform a great service for the queen, thereby saving her honor and virtue. Ultimately, then, d'Artagnan's love and devotion to Constance Bonacieux will be one of the causes for his own advancement in society and will tightly entangle him in the deadly political intrigues of France. In other words, the relationship established here and intensified when d'Artagnan helps Constance Bonacieux slip the duke of Buckingham into the Louvre are sufficient for Constance to trust d'Artagnan to go on the dangerous and highly secret mission for the queen.
The importance of this love affair is a commentary on the times. Dumas writes that Constance Bonacieux was an amorous ideal, that she knew the secrets of the court and was not insensitive to masculine attentions, even though she was married. Furthermore, it was the custom of the time for a young and handsome man to take money or other gifts from his mistress, and the young and handsome d'Artagnan is always in need of money.
The scene where d'Artagnan sees the mysterious woman in a cloak, knocking at what he thinks is Aramis's window is an example of a scene which allows the reader to classify this novel as a "cloak and dagger" novel — that is, mysterious people are often seen half-concealed by cloaks, and they do not reveal themselves until someone has drawn his sword, as does d'Artagnan in this scene.
Chapter 12 presents our first view of George Villiers, the English duke of Buckingham — an extremely handsome and sophisticated man. The love which Buckingham has for the French queen is depicted in terms of his desperate need to be with her. There is no compromise of the queen's honor — except, at the end of the interview, she gives him a gift as a token of her love for him. This gift, a monogrammed, gold-inlaid jewel box made of rosewood, is, as we later discover, filled with diamond tags, or studs, which will become the object of the first real adventure in the novel, when the king demands that the queen wear the jewels to a ball. The king, however, demands that the queen wear the diamonds only because the cardinal tells him that the jewels are in Buckingham's possession; the cardinal wants to prove that the queen is untrue so that he can gain even more power over the king.