Aramis and d'Artagnan are thwarted in their task to transport a young woman and her baby from a village outside Paris to the palace when armed men abduct her baby. They must find out who she is exactly and why anyone would go to such lengths to kidnap her son.

Meanwhile, the king's royal hunting party is disrupted by the sudden appearance of Louis' banished mother, Marie de Medici, seeking royal protection from an unknown assassin. The Musketeers must fight to protect both mother and baby while also loyally defending the throne.

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.