The Duke of Buckingham is impressed that d’Artagnan managed to thwart the cardinal’s attempts to stop him from reaching England. D’Artagnan tells him that he couldn’t have done it without the help of his friends. When the two men reach the duke’s house, he takes d’Artagnan into a room that he’s converted into a shrine for the queen. Among other precious objects, the room contains the diamond tags that the queen gifted him. When the duke looks at the tags, he sees that two of them are missing. He believes they were likely stolen by Lady de Winter (Milady) on behalf of the cardinal.
It is not clear whether the Duke of Buckingham knows about the cardinal’s relationship with the queen. However, the duke makes it clear that he doesn’t like the cardinal, and he is glad d’Artagnan thwarted his plans. Although the duke treats d’Artagnan with kindness and gratitude, he remains a man of questionable morals and motivations. Although he’s obeyed Queen Anne’s orders, he clearly has plans to win her back, and he’s even dedicated a whole room into a shrine for her. Additionally, this passage also sees the return of Milady, who was apparently successful in her undertaking for the cardinal.
However, since there are still several days before the ball, the duke asks his jeweler to fix the diamond tags. The jeweler manages to complete the task in two days by working non-stop and the duke rewards him handsomely. When the studs are finished, the duke thanks d’Artagnan and offers to reward him. However, D’Artagnan feels uncomfortable accepting a reward since France is at war with England. The duke understands d’Artagnan’s reasoning. Nonetheless he insists on helping d’Artagnan get back to Paris as soon as possible. With the duke’s help, d’Artagnan manages to return to Paris quickly. Along the way he is gifted four horses as a present from the duke. The duke expects that he will gift one of the horses to each of his companions and keep the remaining one for himself. As soon as d’Artagnan arrives in Paris, he pays a visit to M. de Tréville who sends him to the Louvre where he can rejoin his company.
Because he is powerful, the duke manages to pull some strings and get the diamond tags fixed in the appropriate amount of time. As long as d’Artagnan can quickly return to London, he should be able to get the queen her tags in time. Even though the duke promises not to give d’Artagnan a gift, he does anyway. His generosity demonstrates that although he can be selfish, he still cares for d'Artagnan and appreciates what he’s done on the queen’s behalf. Of course, the horses are a significant upgrade from the horse d’Artagnan rode to Paris on in the beginning of the story.