Set during the Second World War the story follows six children living in the fictional town of Garmouth which regularly suffers bombing raids by the German Luftwaffe. When Chas McGill finds a crashed German Heinkel 111 bomber he removes a fully operational machine gun and over 2000 rounds of ammunition. With the help of his friends, Cem, Clogger, Carrot Juice, Audrey and Nicky they set up their own den called "Fortress Caporetto", named after a World War I battle in which Chas's grandfather fought.