from the back cover:
Horrible Harry and his classmates all love Halloween. Everyone wears a costume to school - even Miss Mackle! Harry's costumes got scarier and scarier each year. He's been a slithering snake, a slimy green Loch Ness Monster, and a bloody Count Dracula. So everyone in Room 3B is waiting to see what Harry will be this year - and they're all in for a really big surprise!