Following the tragic death of a young girl underneath the royal carriage, The Musketeers' investigations into her missing friend lead them into the world of a highborn and enlightened woman, Ninon, who really gets under Athos' skin. The Musketeers are forced to put aside their animosity towards the Cardinal when his life is placed in danger, and rescue this extraordinary woman.

Queen Anne's taking of the waters is cut short when her life is put in mortal danger. Meanwhile Louis dines with wealthy German banker Count Mellendorf, who is looking for a French match for his beautiful daughter.

As the Musketeers try to spirit Anne back to Paris, they're forced to seek refuge in a fortified convent, where Aramis comes face to face with his past. Under siege and fearful for their lives, the mother superior and nuns help the Musketeers to defend the convent, but what will they do once they run out of ammunition?