The central character, David Elliot, is a 12-year-old seventh son of a seventh son, cruelly mistreated by his parents. His father chooses a series of repressive boarding schools for him, where David does not perform well.
After being expelled from his "last-chance" school, they receive at home the unexpected call from an unknown school, Groosham Grange, of which nobody has heard of before. This school, located on Skrull Island, an island off Norfolk, earnestly invites David to enter it. His father jumps at the chance to pack him off, especially since it appears that the place is suitably severe. He must take a train from Liverpool Street to King's Lynn.
On the train he meets two other new students, Jill Green and Jeffrey Joseph. Along with Jill (seventh daughter of a seventh daughter) and Jeffrey, David anticipates more mistreatment, and makes a pact with them that they will escape at any cost. After they arrive, they start to learn strange things, are baffled by inexplicable disappearances, very peculiar teachers (including a werewolf and a ghost), assorted frights — and an absence of punishments.
They are determined to solve the mystery of the school, but then Jeffrey begins acting suspiciously, seeming to side with the other students rather than with David and Jill...
Slowly, David discovers that he has been recruited to a school for wizards and witches, because he is a wizard himself, and that the staff hopes to help him unleash his full potential as the other students do. David must decide if he will remain at Groosham Grange or not. His dilemma lies in his wish to avoid evil, but the Headmaster tempts him with the promises that he can learn "how to make gold out of lead, how to destroy your enemies just by snapping your fingers... [and] to see into the future and use it for yourself". Of course, the outside world can't ever find out...
Provides examples of:
- Disney Villain Death: Subverted with a school inspector as he is not a villain, but that doesn't stop him from falling to his death.
- Hilariously Abusive Childhood: Happened to David's father, to the point he can't even walk. He somehow fails to see anything wrong with it.
- Join or Die: every student at the Grange, sooner or later, is given this choice.
- Multiple Head Case: The headmasters' office has a sign saying "The Heads", and two voices can be heard from within. David initially makes the sensible assumption that the school has two headmasters, but eventually learns that there's only one headmaster, with two heads.
- Wizarding School: Groosham Grange