A gang extort money from local shopkeepers. So D'Artagnan Jr. and Jacques go undercover to find who is behind this ill deed. Mazarin so finds out what the Musketeers are up and trys to fame them for the deed. Meanwhile Sirco creates the ""sub aquatic chamber"" the world's first submarine and the Musketeers use it to thwart the gang. (thetvdb.com)