High on a rocky crag on the west coast of Scotland stood Whitecrest House, home of the Mackay family of Strathfiddich. Over the centuries its grim, stone walls had looked down on many tales of bravery, mystery and intrigue.

Now, in the Second World War, it was being put to a peaceful purpose – a boarding school for boys. But what the boys didn’t know was that under their feet, in hidden cellars and dungeons, was another kind of school, where the students had names like Kurt…and Hans…and Ernst…