At 2 a.m., the four adventurers, accompanied by their armed servants, ride out of Paris. At the first inn where they stop, Porthos gets into an argument with a stranger; his companions are anxious to be on their way, so they tell him to "kill that man and rejoin us as soon as you can." They continue on their journey and decide to wait two hours for Porthos, but he never appears.
Later, they encounter eight or nine men working on the road, and suddenly the workmen race for the ditch, pick up their muskets, and begin firing. D'Artagnan realizes that they have ridden into an ambush, so he warns the others, urging them back. Mousqueton falls, wounded. Aramis receives wounds and can't ride any farther, so they leave him at an inn in Crevecoeur, tended to by his servant, Bazin.
The original party of eight is now reduced to four: d'Artagnan and his servant Planchet, and Athos and his servant Grimaud. At midnight they reach Amiens and stop at the Lis d'Or inn. Grimaud guards the horses while Planchet sleeps in front of the door so that d'Artagnan and Athos won't be taken by surprise. Two hours later, they are awakened by noises, and at 4 a.m., they hear more loud noises in the stable. They investigate and discover Grimaud lying unconscious with a bleeding head. Planchet goes to saddle the horses but they are still too exhausted to go any farther. Mousqueton's horse has even been bled, mistakenly, by the local veterinarian.
When Athos goes to pay the bill, the innkeeper looks at the money and declares it to be counterfeit. At this moment, four armed men rush toward him, but Athos holds them off while yelling to d'Artagnan to escape.
Outside Calais, both d'Artagnan's and Planchet's horses collapse when they are only a hundred paces from the town gates. They dismount and begin following a young nobleman and his servant. By accident, they overhear a ship's captain stating that he will take no one to England without the written permission of the cardinal. The young nobleman presents a paper signed by the cardinal and is told that the paper must be endorsed by the harbor master.
D'Artagnan and Planchet continue following the two men, pick a quarrel with them, and while Planchet duels with the servant, d'Artagnan duels with the young nobleman. Defeating him, even though he is wounded while doing so, d'Artagnan steals the traveling permit, which is made out to Count de Wardes. He gets the permit signed by the harbor master, takes it to the ship's captain, and he and Planchet sail for England.
For a moment in London, d'Artagnan is at a loss: he knows no English. Nonetheless, he writes the duke of Buckingham's name on a piece of paper and is immediately directed to the duke's residence. The duke's servant, who speaks French, takes d'Artagnan to the field where the duke is hunting with the king. When the duke reads the letter that d'Artagnan gives him, he turns pale and immediately returns to London.
On the ride back to London, d'Artagnan relates his exploits, surprising the duke that someone so young could be so brave, resolute, and resourceful. The duke takes d'Artagnan through many rooms and finally to a concealed chapel, where he shows him a life-sized portrait of the queen of France. Then, as he takes the diamond tags out of their box, he is horrified to see that two of them are missing. He instantly realizes that the ribbons have been cut, and he knows that the diamonds were taken by Milady — Lady de Winter — obviously an agent for the cardinal. Immediately, he sends for his jeweler and his secretary.
He instructs his secretary to have all the English ports closed so that Milady cannot return to France with the diamond tags. When d'Artagnan reveals his astonishment at the duke's enormous, unlimited power and his use of it — all for the sake of his beloved, Anne of Austria, queen of France — Buckingham acknowledges that "Anne of Austria is my true queen. At a word from her, I'd betray my country, my king, even my God." D'Artagnan marvels at such total devotion.
The jeweler arrives and tells Buckingham that duplicating copies of the missing diamonds will take a week; Buckingham offers him double the price if he can finish the job in two days, and he agrees to do so. Since speed is of the utmost importance, the jeweler immediately goes to work in the duke's palace. D'Artagnan is again impressed by the duke's power and his ardent love for the French queen.
After the fake diamond tags are made, the duke wants to reward d'Artagnan, but d'Artagnan reminds the duke that he, d'Artagnan, is serving the queen of France and that some day in the future, he and the duke might be enemies on the battlefield. However, because Buckingham sincerely wants to reward him and because d'Artagnan needs some good horses in order to return to Paris in time for the ball, d'Artagnan accepts four magnificent horses — one for d'Artagnan himself and one for each of the three musketeers. D'Artagnan is also given the secret password that will enable him to change horses. Twelve hours later, he is in Paris.
The next day, all of Paris is talking about the upcoming ball. That night, the king is especially pleased to see that the queen is wearing her diamond tags. The cardinal, however, calls the king's attention to the fact that the queen has only ten tags. He gives two diamond tags to the king and tells him to inquire of the queen about the two tags which are missing. During their next dance together, the king is unable to count the number of diamond tags on his wife, so at the end of the dance, he tells her that two of her tags are missing — and he gives her two more. The queen triumphantly announces that now she has fourteen! The king counts the tags: she is wearing twelve and now she does have two more diamond tags. The cardinal is stunned at the news, but recovers from his astonishment and explains to the king that the two extra diamond tags are his way of making a gift to the queen. Anne is not fooled, however, and she subtly lets the cardinal know that his two diamond tags probably cost him more than the king's original twelve.
Later, d'Artagnan is rewarded for his success in returning the tags; Constance Bonacieux leads him down a series of corridors where the queen presents her hand to be kissed. As d'Artagnan does so, she presses a magnificent ring into his hand. Constance then returns and tells d'Artagnan that she left a note for him at his house.
Again, we can see why this novel is called one of the best swashbuckling, "cloak and dagger" adventure novels. In Chapter 20, the four adventurers embark on a mission and encounter all sorts of unexpected obstacles. Without a doubt, the cardinal seems to be able to know exactly what everyone in the kingdom is doing. Remember that Treville warned d'Artagnan about this very possibility.
On the trip to London, the musketeers and d'Artagnan encounter difficulty at the first inn and leave Porthos. Then during an ambush along the road, they believe that Mousqueton is killed; they know that Aramis is wounded, so they leave him at an inn, tended by his servant, Bazin. Later, Athos is falsely accused by an innkeeper of trying to pass counterfeit money and is attacked by four men. Finally, when d'Artagnan and his servant reach the port of Calais, they discover that the cardinal has had the port closed and is sending one of his men, Count de Wardes, with a special permit to London. Clearly when this novel was written, episodes such as these were truly adventures on the highroads.
The story then continues with d'Artagnan's encounter with the duke of Buckingham and the revelation of the duke's power. Since this novel is also a romantic novel, Dumas's emphasis is often on the power of love. D'Artagnan is in awe of the duke's willingness to use all of his power in the service of his beloved Anne of Austria, queen of France. However, we should remember that this adventure which d'Artagnan undertakes (during which he proves himself to be resolute, brave, and ingenious) is undertaken because of his own devoted love for Constance Bonacieux. Thus we have two plots of love and adventure: one centering on court intrigues; the other, on the romantic intrigues of a daring adventurer and his beloved.
Earlier in the novel, it seemed a superficial scene when d'Artagnan accosted Constance Bonacieux on the Pont-Neuf bridge while she was accompanying a disguised man. Then we learned that the man was the duke of Buckingham; now we can see that Dumas created this unlikely encounter in order for d'Artagnan — a common, foreign soldier — to get an interview with the most powerful man in England. He can identify himself now as "the young man who nearly fought you one night on the Pont-Neuf."
During the queen's encounter with the cardinal, concerning the diamond tags, the cardinal displays his brilliance in the way that he is able to "explain" his motives, but the queen is equally clever; she lets the cardinal know that she is aware of all his secret machinations against her.
These chapters also anticipate future chapters in that we hear more about Milady, a woman who will prove to be the very blackest quintessence of evil, a character responsible for the deaths of many people later in the novel. Likewise, Count de Wardes will also appear later, although in a lesser role than Milady.