City boy Joe doesn't think much will happen to him when he spends the school holidays with his grandparents in the Scottish highlands. But he's in for a surprise almost straightaway: the hills seem moody, the trees shiver as if whispering secrets, and the weather seems to change, in an instant, from clear blue skies to torrential rain. There's a lot to get used to - quite apart from the unfamiliar cows and sheep and old-fashioned ways of living. When Joe makes friends with Robbie, the only other child nearby, he thinks he'll have a friend to depend on, at least. But Robbie proves difficult to get to know, and he soon leads Joe into an even deeper mystery from one hundred and fifty years ago, involving a cache of stolen gold, a dangerous ghost seeking revenge, and a promise that's too valuable to break...