Private detective Albert Campion sets out to plumb the secrets of Saltey, an ancient hamlet on the Essex marshes. A particular transistor, which can amplfy the thoughts and emotions of others so as to appear to be the secret of telepathic commincation, is discovered.  The device works best in younger children who are old enough to understand what is happening but young enough to not have had too much experience.   Somehow this device ends up with two schoolboys – one with a connection to Lady Amanda, Campion’s wife – who are in great danger from those powerful unknown characters (or character) who will do anything to get hold of it.  This setting of Saltey, once the haunt of smugglers, now hides a secret rich and mysterious enough to trap all who enter – and someone in the village is willing to terrorise, murder and raise the very devil to keep that secret to themselves.  Campion has a most difficult challenge to overcome.