Frank and Joe are headed out on a Kentucky spelunking expedition -- exploring caves, that is. But on this particular descent, something stinks to high heaven. Joe stumbles onto an uncharted subcavern, and suddenly the entire expedition is under attack.

The deeper the boys look into the cave, the more hazardous to their health it becomes. They're attacked by a murderous fellow explorer, snared in an ancient booby trap, and caught in a vicious whirlpool. But the Hardys are not about to back down. They're determined to unearth the deadly secret hidden in the cave...even if it takes them to the very bottom of a chasm of danger!