Spies by Michael Frayn

In the peaceful Close, the only visible signs of war are the nightly blackout and a single random bombsite. To the boys, though, the whole district is riddled with secret passages, underground laboratories, and hideaways for secret agents that must be monitored. And then, with six shocking words, Keith reveals that the Germans have infiltrated his family; from that point, the espionage game takes a sinister and unintended turn. A wife's simple errands and a family's ordinary rituals, the unremarkable geography of post office and railway tracks, are no longer the objects of childish speculation but the tragic elements of adult catastrophe.