Tom, Carl, and Bert are spending the summer on the farm. One day they run into a large, surly man with a black monster of a dog. Scout and the black dog are enemies at first sight. The man arouses the boys’ suspicions at once. He is no ordinary vacationer. When Scout pulls a boy from the river, the boys make a friend. From the boy’s mother they hear a story about the abandoned mill and a missing treasure. The story draws them into an adventure that sets the boys on a collision course with the dangerous man and his equally dangerous dog. Trying to play the master detective, Tom leads his friends into a desperate situation. Can they stop Scout from clashing with the powerful black dog as he rushes to their aid? After all their hard work, will the boys lose the treasure after all? Worse yet, will the crook decide to kill the only witnesses to his crime?