Mrs Mannering thinks that a peaceful holiday in the Welsh mountains couldn't possibly land the children in another dangerous adventure. How wrong she is! Philip, Dinah, Lucy-Ann and Jack are soon up to their necks in the mystery of a rumbling mountain, roaming wolves, and a mad genious who plans to rule the world.
What could be more exciting that a holiday in the lonely Welsh mountains, staying at a farm in a place called Doth-goth-oo-elli-othel-in (as enunciated by Bill) with a donkey each to ride about on? For once, Mrs Mannering (or Aunt Allie to Jack and Lucy-Ann) is determined to stick with Bill and the children to keep them out of adventure.