Clarig, on the coast of Sutherland, where Alison and her brother Niall and their friends spend their holidays, provides lots of outdoor fun and adventure at this particular Christmas-time, but there is another mystery to unravel. Is the mysterious Professor's interest in geology genuine? If so, why is his behaviour so suspicious? And why do both Alison and her uncle have a feeling that they have seen him before? Events prove that not rocks, but something very different, are behind the Professor's activities.