A notorious diamond thief and two dim-witted accomplices stop along the highway where 12 year-old Owen sees them mistreating their dog. The boy intervenes to give the thirsty dog a drink, but it escapes into the woods. He helps the angry thugs search for the animal deep into the forest, then ditches them. Owen finds the dog and they hide out in his secret fort, ingeniously fortified with booby traps and defense mechanisms to repel intruders. Bravely, his fort is built atop the ridge where the feared "Madman of the Mountain" is said to live.