The day after d’Artagnan’s meeting with the Cardinal, all of the Musketeers and guards in Paris march off to war. They have to travel to the coast of France to take part in the siege of La Rochelle, a military endeavor that will ultimately become one of the most important efforts of Louis XIII’s reign. The conflict is supposedly about religion, because La Rochelle is a stronghold of Protestants, whereas the French are Catholic. In actual fact, the conflict is about two men’s infatuation with Queen Anne. The Duke of Buckingham brings English forces to fight on the side of La Rochelle because he wants the right to enter France and visit the Queen. Meanwhile, on the French side, the Cardinal hopes to impress the Queen by humiliating Buckingham.

D’Artagnan reaches the outskirts of La Rochelle quite soon, with Monsieur des Essarts and his Guards. Athos, Porthos, and Aramis, as Musketeers, travel with the King, who is detained during the journey due to illness. Because of this, the friends are separated for a few weeks. This leaves d’Artagnan feeling lonely. He has few friends aside from the three Musketeers, so he spends his time alone, thinking about what the Cardinal and Milady may do to him.

One night d’Artagnan goes out to a tavern alone, and on his way home he spots the barrels of two muskets by the side of the road. He ducks just in time to avoid getting shot. As he runs away, a musket ball flies through his hat. Afterward, he examines the hole and realizes it comes from a precision rifle, which is not a weapon that most soldiers or thieves would own. The attack must have been directed at d’Artagnan personally, most likely by Milady.

The following day, d’Artagnan’s commanding officer gives him a chance to lead a small group of men on a reconnaissance mission. Eager to distinguish himself, d’Artagnan accepts this mission gratefully. He asks for volunteers to go along, and he accepts the first four that step forward—two guardsmen and two common soldiers.

D’Artagnan’s job for his reconnaissance mission is to find out how well defended a certain outpost is. Partway there, the two common soldiers slip away from his little group. At first does not think much of this, and he continues on on. The outpost turns out to be well defended, and one of the guardsmen gets killed while the other flees. D’Artagnan is thus left alone and vulnerable. Just then, the two common soldiers begin shooting at him from behind.

Realizing that the soldiers are assassins, d’Artagnan fights them off. One gets killed by crossfire, and d’Artagnan wounds the other. Rather than kill this man, d’Artagnan interrogates him and, because of the information he provides, obtains a letter from Milady. This letter states that Madame Bonacieux has managed to escape her captors and hide herself in a convent somewhere. D'Artagnan is relieved to know that she is safe. 

Back at camp, d’Artagnan is welcomed as a hero. Rather than report the soldier under his command for trying to kill him, he lies and claims that the man fought bravely. The man would have been executed otherwise, so he is now indebted to d’Artagnan.