The brightest and best musketeers of the King's Regiment - Athos, Porthos and Aramis - are dispatched by Captain Treville to find a missing musketeer carrying important letters on behalf of King Louis. Meanwhile, d'Artagnan travels from his family farm in Gascony to Paris to petition the king, but his peaceful plans are turned upside down.

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.