Two English children on a Nazi-occupied Channel Island plan their escape.

Mick and Caroline Templeton were sure that the schooner Polly Harris and its mysterious owner were mixed up in something shady. This mystery made going to school in London to be coached for exams more bearable. Things began to look up when the children discover that they are to live in a place called Tides Reach in a London village, "a very, very old village, and it's right down by the Thames. It's a forgotten village - a secret village." It's called Prettyman's Hard, and the house they are to live in is the oldest in the village. Soon Mick and Caroline are aware of a strangeness, a mystery in the air. Something incredible is behind it all, but it is some time before they discover what it is, after much excitement and danger.