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.