The ancient evil of the worm is pitted against Fliss's courage and determination. Swindells uses the form of a school play to make the supernatural intrude into the ordinary.
The novel is grounded in the tradition of British local legends — the 'worm' (dragon) legends found throughout northern England and Scotland. Swindells takes a real folkloric tradition and makes it literally dangerous.
The children inside the worm gradually lose their own personalities. The horror is the loss of individuality — being subsumed into something else.