As Lloyd and Harvey await the arrival of their new foster sister, they wonder if she will be able to resist the mysterious powers of their sinister headmaster. Based on the books of Gillian Cross.

The boys get suspicious of Dinah's state of mind when she breaks one of the headmaster's rules. Based on Gillian Cross's books.

The Demon Headmaster's plans for Dinah are discovered - and SPLAT forms a counter-plan.