When Sidney Dashworth's school is evacuated to a huge stately home in the countryside, she thinks she's going to spend the war being very bored. At least her brother must be having fun, flying his Spitfire all over France! But soon Sidney and a housemaid called Nancy discover that the Earl is up to no good. He has secret nighttime meetings with mysterious men from the government, is hiding something sinister on his land and is connected to the bombing of a munitions factory. At first it's all terribly thrilling, investigating by creeping about at night and finding secret passageways, but soon everything takes a deadly turn. Sidney's brother goes missing over France and the war hits home with a terrible reality. The Earl and his evil plotters must be stopped, or nothing will ever be the same again.