AmateursleuthsFrankandJoeHardyinvestigateacounterfeitoperationwhiletheirdetectivefatherisconnectedtoasecretcaseinvolvingnationalsecurity.Anoldmillontheoutskirtsoftownappearstoholdthekeytosolvingbothcrimes.Determinedtolearnthesecretoftheoldmill,FrankandJoedeviseacleverschemetogainentrance,onlytofindthemselvestrappedinanundergroundtunnel.RankingontheAll-TimeBestsellingChildren’sBookslist,thethirdinstallmentoftheHardyBoyserieswillhavereadersontheedgeoftheirseats.
