The Duke of Savoy and his wife, the king's sister, arrive in Paris at the same time as Marsac, a former Musketeer, now a deserter, who seeks Aramis's help after an abortive attempt on the duke's life. Marsac's troops were massacred as they slept whilst on a mission to depose the Duke. Marsac's informant tells him they were betrayed to the Duke by Captain Treville, the leader of the Musketeers. Aramis discovers that all records of the night of the massacre have disappeared, whilst the duke suspects that Richelieu was behind it and has located a Spanish spy who can prove it. The Musketeers must prevent the Duke from starting a war with France, as well as proving their captain's innocence.

Frustrated by his wife's infertility, Louis makes a foolish statement, suggesting it were best if she were dead. As a result, Richelieu sends Irish gunman Gallagher and his gang to kill her while she is travelling with the Musketeers. Sending their friends back for reinforcements, Aramis and Athos take her to a convent for sanctuary, where Aramis is surprised to see his former love, Helene is one of the nuns, though she and the Mother Superior prove themselves very capable in the subsequent siege. However, it is with the queen that Aramis grows intimate, to Athos's horror. The siege is lifted, the queen saved and the king contrite whilst Richelieu manages to frame somebody else for the assassination attempt.