London, 1943. Despite the deployment of his father to North Africa, the near constant threat of Nazi air raids, and the day-to-day hardships caused by rationing, thirteen-year-old Derek has managed to withstand the worst of World War II more or less unscathed.The war finally hits home, literally, when a German "land mine" destroys the roof of their house, and Derek and his mother are forced to live with his grandparents. Then Derek discovers that his backyard air raid shelter has been taken over by a suspicious man who claims to be a British double agent. The visitor's true identity and intentions prove to be an irresistible mystery to the schoolboy and his friends.