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?

When they are invited to spend a few weeks with Uncle Arnie at the seashore, little do Tom, Carl, and Bert realize that they will be staying in the hunting lodge of a medieval castle. Uncle Arnie is quite a storyteller. Soon the boys find their imaginations and the lodge peopled with ghosts from Rodensteyn Castle, a castle long ago buried under the sand. But do ghosts leave tracks, tracks that Scout follows into the sea? Are the legends of the ghost of Sir Isobald and the treasure of Rodensteyn Castle true? Is it a ghost that Tom spots in the dunes at night? Can Scout’s nose, no matter how keen, help the boys solve a mystery over 400 years old? Past and present, fact and fiction merge as Tom and his friends search for a treasure that has outlasted the ages.