Simon Peter Richard Ogden, or Sprog to his friends, is on holiday with his aunt in Somerset when he hears about the strange roaring that sometimes disturbs the village. The noise comes from a cave deep in the hills and Sprog discovers that it is nothing less than a dinosaur - with toothache.

Sprog is fed up at the thought of spending Christmas with his Great Aunt Sis in Somerset. Last time he stayed with her he met a dinosaur called Minerva in the caves beneath Mendip. Surely nothing like that could ever happen again - or could it?