Siblings Julian, Dick and Anne Kirrin, their cousin Georgina 'George' and her dog, Timmy, spend a holiday at a coastal farm in Cornwall.

There, they are nicely welcomed and hosted by the talkative Mrs Penruthlan and her enormous husband, whose monosyllabic utterances they find incomprehensible and quite funny.

The children encounter a young boy named Yan (Jan), as well as a group of travelling entertainers called the Barnies.

The children learn that long ago, villainous locals would shine a light on stormy nights to direct ships onto rocks to wreck them, so their cargoes washed ashore and could be stolen.

Julian and Dick discover a light is again being shone at night, so the children set out to solve the mystery.