Polly Flint was the kind of girl who was always "seeing" things, like the angel that one day swept past her in the field. So it was hardly a surprise, at any rate to her, when she began to pick up odd echoes and reflections after she had moved into her aunt's house in the old village. But soon it became clear that these were rather more than the fancies of a lonely girl : somehow, somewhere, the inhabitants of an underground world were calling for help, and she, Polly Flint, was the girl to provide it.