Tania is fourteen years old and lives in London. But this summer she is on holiday at her uncle and aunt's farm in the north-west of England. Her cousin Peter is fifteen. He helps his parents on the farm and shows Tania the countryside. They go walking and swimming in the hills.
One day they see strange things happening at an old farmhouse near the railway line where some Irish tourists are staying. Through Peter's binoculars they watch two men carrying a heavy box up the hill. Then, suddenly, the two men disappear. Where have they gone? And what was in the heavy box? They decide to investigate.