Dinah Glass goes to stay with the Hunter family, and meets her foster brothers Lloyd and Harvey. It's not easy, as they seem to hate her, and the school they all attend is really strange. Pupils talk like robots and do weird things, even Dinah finds herself acting oddly. She's sure the headmaster has some kind of power over them, and is determined to find out more, but the Demon Headmaster is equally determined to stop her!The show is based on Gillian Cross's book series of the same name. The 1st series is based on the first two books 'The Demon Headmaster' and 'The Prime Minister's Brain'. The second series is based on the fourth book 'The Demon Headmaster Strikes Again' and the third series is based on the fifth book, 'The Demon Headmaster Takes Over'. Adapted for television by Helen Cresswell.