from the back cover:
'I don't know that you have done anything wrong,' Miss Hepplewhite said. 'But it is possible that you have done something rather dangerous.'

William and Susie thought they were just playing a game when they cooked a witch's brew in the old barn and said a spell over it, but Martha was not so sure. And indeed, the three friends soon learn that they have called up something dark and evil out of the distant past . . .