The Musketeers are tasked with bringing the fearsome Labarge, a regional intendant under arrest for the murder of two Musketeers, to Paris. The Red Guards, resentful of the Musketeers' handling of him, initiate a vicious fight between the two regiments. Fed up with the disruptive rivalry between the Musketeers and the Red Guards, the king calls a contest between them to settle which one is stronger once and for all. D'Artagnan is determined to be chosen to fight so that he can prove himself worthy to be a Musketeer. But first he has to learn to control his impetuous nature and to fight with his head, not his heart.

The series reaches its thrilling climax as an unexpected rift between the Musketeers threatens to blow them apart. Athos takes a woman hostage in the square, and the Musketeers are amazed to discover he is threatening none other than Milady. Athos is furious when he finds out that D'Artagnan knows her, testing their friendship to its limits. D'Artagnan is forced to do the unthinkable and seeks Milady's help.

The cardinal, fearful the Musketeers will prove he masterminded the attempt on the queen's life, compels Milady to kill them by any means. Milady has no choice but to revisit her dark past, and in doing so puts Constance into danger. The Musketeers must use all their wits and courage to outsmart her and save Constance. Can the Musketeers restore their team and bring Milady and the cardinal to justice? Or will Milady finally wreak her revenge?