When a Spanish general, Tariq Alaman, offers to hand over the formula of a unique and deadly gunpowder in exchange for help rescuing his kidnapped daughter, the Musketeers find themselves tangled in a complex web of political trades with the Spanish. Their mission is further complicated when Porthos is captured by Spanish agents.

Meanwhile, when the dauphin becomes seriously ill, Constance takes matters into her own hands and risks everything to save his life.

When Athos is kidnapped and taken back to his estate of Pinon, the Musketeers go in search of him. They find themselves in the middle of a battle against Baron Renard, who is shocked by Athos' decision to reject his nobility and is determined to seize the land for himself.

As the Musketeers train the people of Pinon in combat and defend them against multiple attacks, Athos is forced to confront his past when he encounters an old friend whose life was ruined by his actions years ago.