Twelve-year-old Toby and his sister Millie, fourteen, are tramping the Heaphy Track with their mother when they go off-track to find an old surveyor's hut their grandfather used. When their mother breaks her leg in a hidden hole the kids set off back to fetch help. They spend some nights alone, hungry and lost. So far, so ordinary, but there is something strange about the cave they've camped next to. A little woman emerges and draws them in with the promise of food and shelter. They enter an underground cavern that is deeper than they first thought and where a whole tribe lives. These people believe in natural law, not human law, and have deliberately hidden away from humans believing that here they can survive a total war or pandemic. The kids are intrigued by the techniques this strange people have used to survive but this is tempered by the growing realisation that the Tribe don't want them to leave...