Assigned to escort privateer Emile Bonnaire from Le Havre to Paris the musketeers have to thwart an escape attempt by his wife Maria and an ambush by his aggrieved ex-business partner Meunier and his men, during which Porthos is wounded, necessitating a sojourn at a deserted château. Athos explains it belongs to his family and was where he married Milady. Bonnaire is revealed to be a slave owner and, on arrival in Paris, bribes Richelieu into forming a partnership operating tobacco plantations in the New World, using slaves as a work force. However there is another faction out to capture Bonnaire and the musketeers are braking no rules in handing him over to them.

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.