Twelve-year-old Klaus spends his vacation with his father, the captain of a border garrison. Klaus and his friend Fritz spend their free time searching the nearby woods for the legendary "Dog in the Moors." 

The only unusual thing the boys find is a friendly sheepdog locked up by himself in the forest. The boys try to bring the dog home, but he suddenly runs off at the sound of a whistle, even though no one else is in sight. After Klaus's father takes an interest in this mystery, the dog turns out to be the clue to an important discovery.