When the three musketeers meet d'Artagnan, they want to go someplace where they cannot be overhead as they make plans. They decide on an inn, but have no privacy there; they are continually bombarded with questions about d'Artagnan's exploits. When they hear some soldiers talking about a bastion that the enemy has taken and temporarily abandoned, Athos makes a bet that they can eat their breakfast there and remain safely in the bastion for one hour. The other soldiers bet against him.
Initially, d'Artagnan, Porthos, and Aramis are perplexed about Athos's ridiculous bet, but are reminded that they need privacy to discuss some very important matters that must remain absolutely secret. In the bastion, they will have complete privacy. Porthos wishes that they had remembered to bring their muskets, but Athos reminds them that when the bastion was stormed, twelve men and their muskets and powder were left lying there. They can use these weapons and receive even greater glory when their colleagues realize what a dangerous mission they went on, theoretically armed only with swords.
In Chapter 47, they enter the bastion, and Athos announces that he saw Milady the previous night. While d'Artagnan is registering surprise, Athos explains to the others what a wicked and evil woman she is and that she tried to have d'Artagnan shot and poisoned during the last two weeks.
Suddenly the musketeers see four soldiers and sixteen workmen approaching. Using the twelve muskets, they take careful aim, killing some of the soldiers and wounding the rest. The workmen flee. Resuming their talk, the musketeers and d'Artagnan decide that they must warn Buckingham against Milady's treachery, but since they arc officially at war with England, they decide to warn Lord de Winter and tell him that he is about to be killed by his sister-in-law and that he should protect himself and Buckingham as well. Their next goal will be discovering the whereabouts of Madame Bonacieux before Milady and the cardinal do. Athos shows them the cardinal's "protection letter" which he took from Milady, signed by the cardinal and insuring absolute protection and permission to the bearer of the note. They decide to send Planchet to London and Bazin to Aramis's countess, but unfortunately, they realize, they need money to carry out their plans.
Grimaud abruptly announces that about twenty-five men are approaching. Athos has Grimaud place all the dead bodies outside and put muskets in their hands. Meanwhile, they finish their breakfast and see that they have probably ten more minutes before they can win their bet. They conceal themselves and carefully take aim at the approaching soldiers, kill several of them, and then, as the rest try to approach the bastion, they push over a rotting wall on them — killing or drowning most of them in the moat. Then they return gloriously to camp.
On the way, they wonder how they will get some money — and at this point they remember d'Artagnan's diamond ring. They convince him that since the queen gave it to him, it would be an honor to the queen if he were to sell it to help Buckingham, the queen's lover, and the money could also help rescue Constance Bonacieux, the queen's loyal servant.
They persuade Aramis to write a letter to de Winter and one to Madame de Chevreuse, using elegant, arabesque phrases so if the letter is confiscated or captured, the enemy (or the cardinal) will not understand the contents. Then they send Planchet and Bazin on the important errands, promising them extra money if they return at a specifically designated time. D'Artagnan has now been officially declared a musketeer, so the four musketeers while away their time, waiting for the servants to return. Not long afterward, both servants return on the designated day at the designated time.
The episode in the bastion does little to advance the plot, but it does emphasize the daring and inventive bravado of the musketeers and d'Artagnan. Of course, the purpose of their going to the bastion, apart from its being a daring excursion, is to find a place where they can discuss secret strategy without being overheard. We know that the cardinal has spies in every nook and cranny of France — if not in all of Europe — therefore, only in a captured and temporarily abandoned bastion can they find sufficient privacy to make plans to thwart Milady's scheme to kill Constance Bonacieux and d'Artagnan.
Athos is characterized in this episode as a true leader: he knows that the muskets belonging to the dead soldiers are still beside their bodies, and by placing the dead men and their muskets on the bastion's parapet, he shows great ingenuity: the attacking soldiers will be firing at dead bodies while the musketeers will be taking direct aim at the attacking soldiers.
Once again, during their discussion about Milady, we realize that she is one of the most villainous and crafty women imaginable. She is totally amoral; she will sacrifice anyone to her deadly, vengeful schemes.
It is interesting to see how several earlier adventures prepared the way for these present adventures. For example, if Planchet had not accompanied d'Artagnan on the trip to England, he would not know how to get there now. Likewise, because Bazin is known by Madame de Chevreuse as Aramis's servant, he will be recognized and trusted.