Great Aunt Emma is no ordinary old lady. But why is she so strange? For a start, she just appeared, grinning, on the doorstep, as if from nowhere. Why have Mum and Dad never mentioned her before—after all, she is supposed to be Granny's sister, isn't she?

Soon Tim and Beth start noticing more and more odd things about the great-aunt they've nicknamed 'Grinny.' And before long, they make a horrifying discovery. She isn't even human, she's as dangerous as a time-bomb and she has a fearful task to perform which involves them…