The tittle of the novel

  The title of the novel  may describe different characters. The most important spies in the story are Keith and Stephen. However, the curious thing is that they become spies because they  believe Keith´s mother is a German spy. So they spy on a spy! They start observing all her moves and following her around. They believe Mrs Haywards may be spying on neighbours in the Close and passing information to the Germans.
At the end of the story, we discover that Mrs Haywards is not a spy at all and that she behaved suspiciously due to the fact that she was having an affair with Uncle Peter, who had deserted from war and was hidden at the Barns. We also found out that the real spy in the Close was the character we least suspected from: Stephen´s father. He was a German Jew who was helping the British side with his knowledge of the German optical industry.