An insight into how Britain, under the threat of German invasion in the Second World War, reinvented itself as a fortified encampment, and how with the Channel no longer a barrier affording any protection, the country turned its attention to the skies. Cruickshank visits Farnborough to see stages in the development of radar, and recalls how Dover witnessed the installation of two huge guns that came to symbolise the defence of Britain.

• Dan Cruickshank