Spies - Michael Frayn

In the quiet cul-de-sac where Keith and Stephen live the only immediate signs of the Second World War are the blackout at night and a single random bombsite. But the two boys start to suspect all is not as it seems when one day Keith announces a disconcerting discovery: the Germans have infiltrated his own family. And when the secret underground world they have dreamed up emerges from the shadows they find themselves engulfed in mysteries far deeper and more painful than they had bargained for.