While Tom and his two friends are making plans for a vacation, Carl’s father announces that he has acquired a sailboat, as part of a business deal. The three boys see the chance for a very unusual holiday.

Their peaceful sailing trip is sabotaged when they run into a gang of notorious carnival followers who put on sidewalk shows as a cover for more profitable sidelines. After a confrontation with the gang, the three sailors and Scout find themselves in trouble with the police. Their attempt to clear themselves leads to a wild chase through rivers, canals, and lakes. When at last the boys come face to face with the gang and its ruthless leader, they are stranded on an island in the middle of an isolated lake. In the showdown they are all alone — except for Scout.

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.