Danny Leach's diamond is worth millions. His father has left it for him. The trouble is that his father has hidden it somewhere in the Monster Tunnels. The good thing is that Danny has Silas Potts and some of the Potty People to help him find it. There's a key and there's a puzzle that will lead to the diamond, but the key and the puzzle themselves have to be found first. Danny's also got his friends at the Potty People's school, both boys and girls, to help him. The Potty People believe that life should be fun, and at their school, with its potty teachers, the kids have fun while they learn.
Danny's Uncle Archie would like to get his hands on the diamond for himself, and his evil stepmother, Caramella, is determined to have it. Everyone must be wary of the mechanical creatures in the Monster Tunnels. People say there are ghosts in there, too. The clues to finding the diamond are there for anyone to see and solve. Danny wants a proper home more than he wants the diamond. Will he find that, as well?