During the height of World War II, young friends Tony, Ursula and young brother Jimmy, play and explore the ancient woodlands across the road from where they are neighbours. Suddenly, an ear-splitting noise rends the air above them! Enemy planes are flying low over them towards London and as they come to their area, and with fear of impending invasion, they regularly take refuge in the woods where they build a den, hoping to hide from it all. During this upheaval, two strangers move into their neighbourhood. One has a car. With most cars off the road, this piques their curiosity. He is unfriendly and acts very secretively, coming and going at odd times. They call him Mr. X and become amateur detectives to investigate his secrets, watching the house and following him on their bikes. The other stranger is French with a bad limp and a walking stick. He also fascinates them. Ursula befriends him on his daily exercise walks and, using her charm, gradually learns about his background. Surprisingly, he reveals he is in French Intelligence and is looking for a murderer living locally. As the war progresses, the trio spend all their spare time monitoring both of them to discover what they are up to. Jimmy is convinced that Mr. X is a spy and bets on it. They discover both are carrying dark secrets from pre-war France that are now playing out in their locality. Constantly searching for answers, they piece together a picture which connects the two strangers. In the final months of the war, the truth suddenly emerges which involves two arrests and one release. To our trio this comes as a real surprise.