It's Halloween at Penhallow Hall and while celebrations are in full swing at the stately home, Polly and Rex are in search of a new ghost dog. Down in the old stable block they come across a dog's head carved in a wooden door and wake a terrier called Patch. The little dog is terrified of the changes to his old home and runs off. When Polly and Rex find Patch hiding in a hollow tree they find themselves on the trail of an even greater mystery. Inside the tree is a coded message from Patch's master, Jack, to his older brother. From Patch's description Polly and Rex realize that Jack's brother must have been a highwayman! But the message remains unread. Can Polly and Rex discover what happened to the brothers?