King Louis tasks the Musketeers with bringing his exiled brother, the Duke of Orléans, back to Paris in order to mend bridges. On the way, the Duke is robbed of dangerous letters which could destroy Orléans. The Duke informs the corrupt Governor of this, and the Governor's Red Guard must race the Musketeers in the hunt for the thief.

The Musketeers battle to restore order in Paris after Grimaud and Feron stage an ambitious Châtelet breakout, setting free a lunatic with murderous designs on the Louvre.